Leave a shiny iron nail outside for a month and it comes back coated in crumbly orange rust. The rust is not iron — so where did the new stuff come from?
You've already sorted changes into physical and chemical. This lesson gives the chemical kind its precise name and its defining feature.
The idea
In a 'chemical reaction', one or more starting substances change into one or more new substances.
The new substances have properties different from the starting substances.
Iron combining with oxygen to form rust is a chemical reaction.
Rust is a new substance — orange and crumbly, where iron is gray, hard, and strong.
Every chemical change you met in earlier grades is a chemical reaction — the reaction is the event that makes the new substances.
Worked examples
Worked example 1. Wood burns in a campfire, leaving ash and giving off smoke. Why is burning a chemical reaction?
Step 1
Answer: the ash and smoke are new substances, with properties different from the wood.
Worked example 2. What must a change produce to count as a chemical reaction?
Step 1
Answer: one or more new substances, with properties different from the starting substances.
You can now state that in a chemical reaction, one or more starting substances change into one or more new substances with different properties.
Check your understanding
Which statement describes what happens in every chemical reaction?
AOne or more starting substances change into one or more new substances.correct
BThe starting substances change state, from solid to liquid or liquid to gas.
This option is wrong — you described a state change — a state change keeps the same substance, while a reaction produces new substances.
CThe starting substances break into smaller pieces of the same substances.
This option is wrong — you described a physical change of size — grinding or breaking leaves the substance itself unchanged.
DThe starting substances mix together evenly without changing.
This option is wrong — you described mixing — a mixture still contains the original substances, while a reaction produces new ones.
A chemical reaction is defined by what it produces. One or more starting substances change into one or more new substances. The new substances have properties different from the starting substances.
Check your understanding
Over years, a copper roof's shiny brown surface turns into a dull green coating. The green coating is a different substance from copper. What kind of change is this?
AA chemical reactioncorrect
BA change of state
This option is wrong — you treated the green coating as copper in another state — a state change keeps the same substance, and the coating is a different substance.
CA change of shape
This option is wrong — you looked at the roof's form — its shape never changed, and shape changes make no new substance anyway.
DMixing
This option is wrong — you pictured the coating as something stirred onto the roof — the coating formed FROM the copper, as a new substance.
Ask the defining question: did a new substance form? The green coating is a different substance from copper, with different properties. A change that produces a new substance is a chemical reaction.
Check your understanding
A cook cracks a raw egg into a hot pan, and the clear runny part turns into a white rubbery solid with different properties. What shows that a chemical reaction happened?
AA new substance with different properties formed.correct
BThe egg got hotter in the pan.
This option is wrong — you took heating itself as the proof — heating without a new substance is a physical change.
CThe egg spread out and changed shape.
This option is wrong — you used the shape change — shape changes happen in physical changes too, and they make no new substance.
DThe egg touched the hot metal of the pan.
This option is wrong — you used contact as the test — touching proves nothing; the test is whether a new substance formed.
The defining feature of a chemical reaction is its product. The white rubbery solid is a new substance, with properties different from the clear runny egg. A new substance with different properties is what marks a chemical reaction.
Lesson 2 of 51 · RXN-002
Why new substances form
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You've seen that a chemical reaction produces new substances. The new substances have to be made of something — and nothing new is delivered.
The idea
New substances form in a chemical reaction because the atoms themselves are not created, destroyed, or changed — they only separate and regroup into new combinations.
Every atom present after a reaction was already there before it.
Burn carbon in oxygen, and the carbon atoms and oxygen atoms regroup into carbon dioxide.
Carbon dioxide is a new substance, but every one of its atoms is an old atom in a new combination.
The particle diagram shows the same atoms before and after — only the grouping changes.
Same 2 carbon atoms and 4 oxygen atoms on both sides — only the grouping changes.
Worked examples
Worked example 1. Iron and sulfur are heated together and form iron sulfide, a new substance. Where did the atoms in the iron sulfide come from?
Step 1
Every iron atom and every sulfur atom in the iron sulfide was present before the heating.
Step 2
The same iron and sulfur atoms regrouped into iron sulfide — the reaction created no atoms.
Worked example 2. Hydrogen burns in oxygen, forming water. A student says the reaction destroyed the hydrogen. What actually happened to the hydrogen atoms?
Step 1
The hydrogen the student saw disappear was the SUBSTANCE, not the atoms.
Step 2
Nothing destroyed the hydrogen atoms — they regrouped with oxygen atoms into water molecules.
You can now explain that new substances form in a chemical reaction because the atoms themselves are not created, destroyed, or changed — they only separate and regroup into new combinations.
Check your understanding
Magnesium burns in oxygen, forming magnesium oxide, a new white substance. Why does a new substance form?
AThe magnesium and oxygen atoms regrouped into a new combination.correct
BThe magnesium atoms changed into different kinds of atoms.
This option is wrong — you changed the atoms themselves — atoms keep their identity; only their combinations change.
CNew atoms were created as the magnesium burned.
This option is wrong — you created atoms — a reaction creates no atoms; every atom in the product was there before.
DThe oxygen atoms were destroyed and replaced by white ones.
This option is wrong — you destroyed atoms — no atoms are destroyed; the white color belongs to the new combination, magnesium oxide.
New substances form because the atoms themselves are not created, destroyed, or changed — they only separate and regroup into new combinations. The magnesium and oxygen atoms regrouped into magnesium oxide. A new combination of old atoms is a new substance.
Check your understanding
Water is split by electricity into hydrogen and oxygen. Where do the hydrogen atoms in the hydrogen gas come from?
AThey were inside the water molecules all along.correct
BThe electricity created them.
This option is wrong — you let electricity create atoms — electricity only drives the regrouping; no atoms are created.
COxygen atoms changed into hydrogen atoms.
This option is wrong — you changed one kind of atom into another — atoms are not changed in a chemical reaction.
DThey came out of the container's walls.
This option is wrong — you imported atoms from outside — every hydrogen atom was already in the water.
The atoms themselves are not created, destroyed, or changed — they only separate and regroup into new combinations. Each water molecule already held hydrogen atoms and an oxygen atom. Splitting water separates those same atoms into hydrogen gas and oxygen gas.
Check your understanding
In any chemical reaction, what happens to the atoms?
AThey separate and regroup into new combinations.correct
BSome are destroyed and new ones are created in their place.
This option is wrong — you destroyed and created atoms — a chemical reaction does neither.
CThey change into atoms of different elements.
This option is wrong — you changed the atoms' identity — each atom stays the element it was.
DThey disappear during the change and return afterward.
This option is wrong — you let atoms blink out of existence — the atoms are present the whole time, regrouping.
The atoms themselves are not created, destroyed, or changed. They only separate and regroup into new combinations. The new combinations are the new substances.
Lesson 3 of 51 · RXN-003
Signs of a chemical reaction
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Did You Know?
Atoms regroup far too small to watch. At the bench, chemists watch for what the regrouping does to the mixture in front of them.
The idea
Five observable signs tell you a chemical reaction may have occurred.
A gas forms — bubbles rise from a mixture that is not being boiled.
A solid forms when two solutions are mixed.
The color changes unexpectedly.
The temperature changes — the mixture gets hotter or colder on its own.
Light is given off.
Each sign says a reaction MAY have occurred — the signs are clues, and the next lesson weighs how much they prove.
Signs a chemical reaction may have occurred
Sign
What you observe
A gas forms
bubbles rise from a mixture that is not being boiled
A solid forms
a solid appears when two solutions are mixed
The color changes
the color changes unexpectedly
The temperature changes
the mixture gets hotter or colder on its own
Light is given off
the mixture shines with its own light
Each sign is a clue that a reaction may have occurred.
Drop a tablet into water and it fizzes: the fizzing is a gas forming, a sign that a reaction may have occurred.
Worked examples
Worked example 1. A glow stick is snapped and begins to shine. Which sign that a reaction may have occurred is this?
Step 1
The stick gives off its own light.
Step 2
Light is given off.
Worked example 2. Two clear solutions are poured together, and the mixture instantly turns cloudy as a yellow solid appears. Which sign is this?
Step 1
A solid appeared where two liquids were mixed.
Step 2
A solid forms when two solutions are mixed.
You can now identify the observable signs that a chemical reaction may have occurred: a gas forms, a solid forms when solutions are mixed, the color changes unexpectedly, the temperature changes, or light is given off.
Check your understanding
A camper squeezes a hand warmer, and the pack grows hot in her hands. Which sign that a reaction may have occurred is this?
AThe temperature changescorrect
BA gas forms
This option is wrong — you reported a sign the scene never shows — nothing bubbled; the pack grew hot, which is a temperature change.
CThe color changes
This option is wrong — you reported a sign the scene never shows — no color changed; the pack grew hot, which is a temperature change.
DLight is given off
This option is wrong — you confused heat with light — the pack shines no light; it grew hot, which is a temperature change.
Match the observation to the sign. The pack grew hot on its own — the mixture's temperature changed. The sign is: the temperature changes.
Check your understanding
A geologist drips a test liquid onto a chip of limestone, and the drop instantly fizzes with bubbles. Which sign that a reaction may have occurred is this?
AA gas formscorrect
BA solid forms when two solutions are mixed
This option is wrong — you reported the wrong sign — the limestone was already solid; the new thing is the bubbles, which are a gas forming.
CThe temperature changes
This option is wrong — you reported a sign the scene never shows — nothing about heat is observed; the bubbles are a gas forming.
DLight is given off
This option is wrong — you reported a sign the scene never shows — nothing glows; the bubbles are a gas forming.
Match the observation to the sign. Bubbles rise from a drop that nobody is boiling. The sign is: a gas forms.
Check your understanding
A drop of a cleaning liquid lands on a red juice stain, and the red patch slowly turns white. Which sign that a reaction may have occurred is this?
AThe color changescorrect
BA gas forms
This option is wrong — you reported a sign the scene never shows — nothing bubbled; the red turning white is an unexpected color change.
CA solid forms when two solutions are mixed
This option is wrong — you reported a sign the scene never shows — no solid appeared; the red turning white is an unexpected color change.
DThe temperature changes
This option is wrong — you reported a sign the scene never shows — nothing about heat is observed; the red turning white is an unexpected color change.
Match the observation to the sign. The stain's red turned white with no dye added or removed by hand. The sign is: the color changes unexpectedly.
Lesson 4 of 51 · RXN-004
Signs are evidence, not proof
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You've matched observations to the five reaction signs. Now comes the harder judgment: how much does a sign actually prove?
The idea
Every one of the five signs can also be produced by a change that is not a chemical reaction.
Bubbles rise from boiling water, but boiling only turns liquid water into water gas — no new substance forms.
So a sign is evidence that a reaction may have occurred — it is not proof.
To settle the question, chemists test whether a new substance with different properties is really there.
Judge each observation by asking: could this happen without any new substance forming?
Worked examples
Worked example 1. A puddle of water disappears from the sidewalk on a hot day. Does the disappearance prove a chemical reaction occurred?
Step 1
Ask: could this happen without a new substance forming?
Step 2
Evaporation turns liquid water into water vapor — the same substance, spread into the air.
Step 3
No — the disappearance is a state change, not proof of a reaction.
Worked example 2. A spoonful of white powder is stirred into water, and the mixture grows noticeably colder. Does the temperature change prove a reaction?
Step 1
Ask: could this happen without a new substance forming?
Step 2
Dissolving alone can change a mixture's temperature.
Step 3
No — the powder may simply have dissolved; the temperature change is evidence, not proof.
You can now judge whether an observation guarantees that a chemical reaction occurred, using the fact that reaction signs are evidence rather than proof.
Check your understanding
Frost forms on a car windshield overnight. A student says the white coating proves a chemical reaction occurred. What is the right judgment?
ANo reaction is proven — the frost is water changing state, not a new substance.correct
BThe student is right — a new white coating always means a new substance.
This option is wrong — you treated appearance as proof — frost is water vapor freezing, the same substance in a new state.
CThe student is right — cold alone can drive chemical reactions.
This option is wrong — you argued from the cold — whether cold can drive reactions is beside the point, because frost is just frozen water.
DNo reaction is proven, because coatings never come from reactions.
This option is wrong — you overcorrected — some coatings, such as rust, ARE new substances; the reason here is that frost is water in another state.
Ask: could this happen without a new substance forming? Frost is water vapor freezing into solid water — the same substance in a new state. So the coating is not proof of a chemical reaction.
Check your understanding
A red drink powder is stirred into water, and the water turns red. Does the color change prove a chemical reaction?
ANo — the powder's own color spread through the water as it dissolved.correct
BYes — any color change is proof of a reaction.
This option is wrong — you promoted a sign to proof — a sign is evidence, and here the color simply spread as the powder dissolved.
CYes — dissolving is itself a chemical reaction.
This option is wrong — you classified dissolving as a reaction — dissolved powder is still the same substance, spread through the water.
DNo — color changes never accompany chemical reactions.
This option is wrong — you overcorrected — unexpected color changes ARE a reaction sign; this one just has a physical explanation.
Ask: could this happen without a new substance forming? The red is the powder's own color, carried through the water as it dissolves. A color change with a physical explanation proves no reaction.
Check your understanding
Which conclusion fits ANY single reaction sign, such as bubbles or a temperature change, on its own?
AA reaction may have occurred — the sign is evidence, not proof.correct
BA chemical reaction definitely occurred.
This option is wrong — you treated a sign as proof — every sign can also come from a change that makes no new substance.
CA chemical reaction definitely did not occur.
This option is wrong — you flipped the sign's meaning — a sign counts FOR a reaction; it just does not settle the question.
DThe sign tells you nothing either way.
This option is wrong — you threw the evidence away — a sign is real evidence that a reaction may have occurred; it is only proof that is missing.
Every sign can be produced by a change that is not a chemical reaction. So one sign alone is evidence that a reaction may have occurred. Proof needs a test showing a new substance with different properties.
Lesson 5 of 51 · RXN-005
Reactants and products
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Wonder this:
Writing 'iron and sulfur are heated together and react to form iron sulfide' takes eleven words. Chemists write the same event in five symbols.
You've read chemical formulas since the bonding units. A chemical equation puts formulas to work describing a whole reaction.
The idea
A 'chemical equation' is chemistry's shorthand for a reaction.
The starting substances are written on the left of an arrow, and they are called the 'reactants'.
The new substances formed are written on the right of the arrow, and they are called the 'products'.
The arrow → reads as 'react to form'.
A plus sign separates substances on the same side.
In Fe + S → FeS, iron and sulfur are the reactants, and iron sulfide, FeS, is the product.
Worked examples
Worked example 1. In the equation C + O₂ → CO₂, name the reactants and the product.
Step 1
The substances left of the arrow are the reactants: C and O₂.
Step 2
The substance right of the arrow is the product: CO₂.
Step 3
Reactants: carbon and oxygen. Product: carbon dioxide.
Worked example 2. In the equation CaCO₃ → CaO + CO₂, name the reactant and the products.
Step 1
Only one substance sits left of the arrow, so there is one reactant: CaCO₃.
Step 2
Two substances sit right of the arrow, so there are two products: CaO and CO₂.
Step 3
Reactant: calcium carbonate. Products: calcium oxide and carbon dioxide.
You can now identify the reactants and products in a chemical equation, reading the arrow as 'react to form'.
Check your understanding
In the equation Zn + S → ZnS, which substances are the reactants?
AZn and Scorrect
BZnS
This option is wrong — you picked the product — reactants sit on the LEFT of the arrow.
CZn and ZnS
This option is wrong — you mixed one substance from each side — both reactants sit left of the arrow, and ZnS is the product.
DS and ZnS
This option is wrong — you mixed one substance from each side — both reactants sit left of the arrow, and ZnS is the product.
Reactants are the starting substances, written left of the arrow. Left of the arrow: Zn and S. ZnS, right of the arrow, is the product.
Check your understanding
In the equation CuCO₃ → CuO + CO₂, which substances are the products?
ACuO and CO₂correct
BCuCO₃
This option is wrong — you picked the reactant — products sit on the RIGHT of the arrow.
CCuCO₃ and CuO
This option is wrong — you mixed one substance from each side — both products sit right of the arrow, and CuCO₃ is the reactant.
DCO₂ only
This option is wrong — you dropped a product — everything right of the arrow is a product, and there are two: CuO and CO₂.
Products are the new substances, written right of the arrow. Right of the arrow: CuO and CO₂. CuCO₃, left of the arrow, is the reactant.
Check your understanding
How does the arrow in a chemical equation read?
A'React to form'correct
B'Is equal to'
This option is wrong — you read the arrow as an equals sign — the two sides are different substances, not equal amounts of the same thing.
C'Is mixed with'
This option is wrong — you gave the arrow the plus sign's job — the plus sign separates substances on one side; the arrow marks the change.
D'Turns back into'
This option is wrong — you reversed the direction — the reactants on the left form the products on the right.
The arrow marks the change from starting substances to new substances. It reads 'react to form'. Reactants → products.
Lesson 6 of 51 · RXN-006
State symbols
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An equation so far names the substances but not their forms. Salt as crystals and salt dissolved in water behave very differently, and equations record the difference.
The idea
A 'state symbol' is a label in parentheses after a formula that tells you the substance's physical form.
(s) means solid.
(l) means liquid.
(g) means gas.
(aq) means 'aqueous' — dissolved in water.
NaCl(aq) reads as sodium chloride dissolved in water.
State symbols
Symbol
Meaning
(s)
solid
(l)
liquid
(g)
gas
(aq)
aqueous — dissolved in water
A state symbol after a formula tells you the substance's physical form.
NaCl(s) is the same compound as table-salt crystals — the state symbol is the only difference.
Worked examples
Worked example 1. What does the (g) in CO₂(g) tell you?
Step 1
Answer: the carbon dioxide is a gas.
Worked example 2. What does the (aq) in KNO₃(aq) tell you?
Step 1
Answer: the potassium nitrate is dissolved in water.
You can now state the meaning of the state symbols (s), (l), (g), and (aq) in a chemical equation, where (aq) means dissolved in water.
Check your understanding
In the equation Mg(s) + FeCl₂(aq) → MgCl₂(aq) + Fe(s), what does the (s) after Mg mean?
AThe magnesium is a solid.correct
BThe magnesium is dissolved in water.
This option is wrong — you read (s) as 'solution' — dissolved in water is (aq); (s) means solid.
CThe magnesium is a gas.
This option is wrong — you swapped symbols — gas is (g); (s) means solid.
DThe magnesium is a liquid.
This option is wrong — you swapped symbols — liquid is (l); (s) means solid.
Read the state symbol in the parentheses. (s) means solid. Mg(s) is solid magnesium.
Check your understanding
Which state symbol shows a substance dissolved in water?
A(aq)correct
B(l)
This option is wrong — you picked liquid — a dissolved substance sits in liquid water, but the label for dissolved is (aq).
C(s)
This option is wrong — you matched the letter s to 'solution' — (s) means solid.
D(g)
This option is wrong — you picked gas — (g) means gas; dissolved in water is (aq).
(aq) stands for aqueous. Aqueous means dissolved in water. (l) is reserved for a pure liquid, such as water itself.
Check your understanding
A teaspoon of potassium chloride, KCl, is stirred into a glass of water until every grain disappears. Which state symbol now belongs after KCl?
A(aq)correct
B(l)
This option is wrong — you called the dissolved salt a liquid — it is dissolved in water, which is (aq).
C(s)
This option is wrong — you kept the starting state — once dissolved, the potassium chloride is (aq), not (s).
D(g)
This option is wrong — you read the disappearance as becoming a gas — the salt is dissolved in the water, which is (aq).
The grains disappeared because the potassium chloride dissolved in the water. Dissolved in water is aqueous. KCl(aq).
Lesson 7 of 51 · RXN-007
Word equation to formula equation
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Did You Know?
A reaction written with substance names is a 'word equation'. You've already written formulas from names in the bonding units — translating a word equation just runs that skill across a whole reaction.
The idea
To translate a word equation into a 'formula equation', swap each name for its chemical formula and keep everything else in place.
Write ionic formulas from the ion charges, and molecular formulas from the prefixes — the routines from the bonding units.
An element on its own is written as its symbol, such as Fe for iron.
Seven elements are two-atom molecules when uncombined — hydrogen, nitrogen, oxygen, fluorine, chlorine, bromine, and iodine, written H₂, N₂, O₂, F₂, Cl₂, Br₂, I₂; you've seen H₂, N₂, O₂, and Cl₂ written this way in the covalent-bonding unit.
Keep the plus signs and the arrow exactly where the words had them.
iron + sulfur → iron(II) sulfide becomes Fe + S → FeS.
Worked examples
Worked example 1. Translate into a formula equation: calcium oxide + water → calcium hydroxide.
Step 1
calcium oxide: Ca²⁺ and O²⁻ cancel one-to-one, giving CaO.
Step 2
water is H₂O.
Step 3
calcium hydroxide: Ca²⁺ and OH⁻ give Ca(OH)₂.
Step 4
CaO + H₂O → Ca(OH)₂
Worked example 2. Translate into a formula equation: sulfur + oxygen → sulfur dioxide.
Step 1
Sulfur on its own is S.
Step 2
Oxygen gas is the two-atom molecule O₂.
Step 3
Sulfur dioxide — di- means two oxygens — is SO₂.
Step 4
S + O₂ → SO₂
You can now translate a word equation into a formula equation using correct chemical formulas.
Check your understanding
Translate into a formula equation: magnesium + sulfur → magnesium sulfide.
Accepted answer: Mg + S → MgS
magnesium: the element on its own is Mg. sulfur: the element on its own is S. magnesium sulfide: Mg²⁺ and S²⁻ cancel one-to-one, giving MgS. Mg + S → MgS
Check your understanding
Which formula equation translates 'copper(II) oxide + carbon monoxide → copper + carbon dioxide'?
ACuO + CO → Cu + CO₂correct
BCu₂O + CO → Cu + CO₂
This option is wrong — you wrote copper(I) oxide — the (II) says Cu²⁺, which cancels O²⁻ one-to-one as CuO.
CCuO + CO₂ → Cu + CO
This option is wrong — you swapped monoxide and dioxide — mono- is one oxygen, CO; di- is two, CO₂.
DCuO + CO → CuO₂ + C
This option is wrong — you rebuilt the products — copper is the bare element Cu, and carbon dioxide is CO₂.
copper(II) oxide: Cu²⁺ and O²⁻ give CuO. carbon monoxide is CO; carbon dioxide is CO₂ — the prefixes count the oxygens. copper on its own is Cu. CuO + CO → Cu + CO₂
Check your understanding
Translate into a formula equation: zinc oxide + sulfur trioxide → zinc sulfate.
Accepted answer: ZnO + SO₃ → ZnSO₄
zinc oxide: Zn²⁺ and O²⁻ give ZnO. sulfur trioxide: tri- means three oxygens, SO₃. zinc sulfate: Zn²⁺ and SO₄²⁻ cancel one-to-one, giving ZnSO₄. ZnO + SO₃ → ZnSO₄
Lesson 8 of 51 · RXN-008
Formula equation to word equation
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Did You Know?
Translation runs the other way too: a formula equation becomes a word equation when every formula gets its correct name.
The idea
To translate a formula equation into a word equation, swap each formula for its correct name and keep everything else in place.
Name ionic compounds by their ions — with a Roman numeral when the metal forms more than one ion.
Name molecular compounds with prefixes, the routine from the covalent-bonding unit.
An element's formula is named as the element itself, such as Cu for copper.
This option is wrong — you used prefix names on ionic compounds — prefixes belong to molecular compounds; the ionic names are zinc chloride and magnesium chloride.
This option is wrong — you swapped the sides — the reactants Mg and ZnCl₂ must stay left of the arrow.
Mg is magnesium; Zn is zinc. ZnCl₂ is zinc chloride, and MgCl₂ is magnesium chloride — ionic names take no prefixes. magnesium + zinc chloride → magnesium chloride + zinc
This option is wrong — you picked the wrong Roman numeral — one CO₃²⁻ balances Pb²⁺, so the metal is lead(II).
PbCO₃: one CO₃²⁻ balances Pb²⁺, so the name is lead(II) carbonate. PbO: one O²⁻ balances Pb²⁺ — lead(II) oxide. CO₂ is carbon dioxide. lead(II) carbonate → lead(II) oxide + carbon dioxide
Check your understanding
Which word equation translates Ni + CuSO₄ → NiSO₄ + Cu?
This option is wrong — you swapped the sides — the reactants Ni and CuSO₄ must stay left of the arrow.
Ni is nickel; Cu is copper. CuSO₄: one SO₄²⁻ balances Cu²⁺ — copper(II) sulfate; NiSO₄ is nickel(II) sulfate. nickel + copper(II) sulfate → nickel(II) sulfate + copper
Lesson 9 of 51 · RXN-009
Coefficients vs subscripts
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Did You Know?
Equations often need to say 'two molecules of this'. There is a right way and a disastrous way to write that.
The idea
A 'coefficient' is a number written in front of a whole formula.
A coefficient multiplies everything in the formula after it.
2H₂O means two complete water molecules.
A subscript lives inside the formula — and changing a subscript changes what the substance IS.
H₂O₂ is not two waters — it is hydrogen peroxide, a different substance.
Change a coefficient and you change the AMOUNT; change a subscript and you change the SUBSTANCE.
The coefficient in 2H₂O makes two of the same molecule; the subscript in H₂O₂ makes a different substance.
Worked examples
Worked example 1. What does 3CO₂ represent?
Step 1
The coefficient 3 multiplies the whole formula CO₂.
Step 2
Three complete carbon dioxide molecules.
Worked example 2. SO₂ becomes SO₃ in one rewrite and 2SO₂ in another. Which rewrite changed the substance?
Step 1
SO₂ → SO₃ changes a subscript, so the substance changes — sulfur dioxide became sulfur trioxide.
Step 2
SO₂ → 2SO₂ adds a coefficient — the same substance, two molecules of it.
Step 3
The subscript rewrite changed the substance; the coefficient rewrite only changed the amount.
You can now distinguish the effect of changing a coefficient, the number written before a formula that multiplies the whole formula, from the effect of changing a subscript, which turns the formula into a different substance.
Check your understanding
What does 4NH₃ represent?
AFour complete ammonia moleculescorrect
BOne molecule containing four nitrogen atoms
This option is wrong — you pushed the coefficient inside the molecule — the 4 in front counts whole NH₃ molecules.
CA different substance from NH₃
This option is wrong — you treated the coefficient like a subscript — a coefficient changes the amount, never the substance.
DFour nitrogen atoms and three hydrogen atoms
This option is wrong — you read the 4 and the 3 as separate atom counts — the 4 multiplies the entire formula NH₃.
A coefficient multiplies everything in the formula after it. 4NH₃ is four complete ammonia molecules. The substance is unchanged — only the amount grew.
Check your understanding
Which change turns carbon monoxide, CO, into a different substance?
ARewriting it as CO₂correct
BRewriting it as 2CO
This option is wrong — you treated a coefficient as identity-changing — 2CO is still carbon monoxide, two molecules of it.
CRewriting it as 3CO
This option is wrong — you treated a coefficient as identity-changing — 3CO is still carbon monoxide, three molecules of it.
DRewriting it as CO(g)
This option is wrong — you treated a state symbol as identity-changing — (g) records the physical form of the same substance.
Changing a subscript changes what the substance is. CO → CO₂ turns carbon monoxide into carbon dioxide, a different substance. Coefficients and state symbols leave the substance itself unchanged.
Check your understanding
What is the difference between 2NO and N₂O?
A2NO is two molecules of one substance; N₂O is one molecule of a different substance.correct
BThey mean exactly the same thing, just written in two different ways.
This option is wrong — you equated a coefficient with a subscript — the 2 in 2NO counts molecules, while the ₂ in N₂O builds a different molecule.
CBoth are two molecules of nitrogen monoxide.
This option is wrong — you read N₂O's subscript as a molecule count — N₂O is a single molecule holding two nitrogen atoms.
DBoth are one molecule, drawn at different sizes.
This option is wrong — you dropped the coefficient — the 2 in 2NO means two complete molecules.
The 2 in 2NO is a coefficient: two complete NO molecules. The ₂ in N₂O is a subscript: one molecule with two nitrogen atoms — a different substance. Coefficient: amount. Subscript: identity.
Lesson 10 of 51 · RXN-010
Counting atoms with coefficients
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You've counted the atoms in a single formula unit. A coefficient scales that count, and the multiplication must be exact.
The idea
To count atoms of an element, first count them inside ONE formula unit — subscripts, parentheses and all.
Then multiply that count by the coefficient.
In 3Ca(OH)₂, one formula unit holds 2 oxygen atoms, because the subscript 2 outside the parentheses doubles the OH inside.
One formula unit holds 2 oxygen atoms; the coefficient 3 makes 3 × 2 = 6.
The coefficient multiplies every element in the formula — 3Ca(OH)₂ also represents 3 calcium atoms and 6 hydrogen atoms.
Worked examples
Worked example 1. How many hydrogen atoms does 4NH₃ represent?
Step 1
One NH₃ molecule holds 3 hydrogen atoms.
Step 2
Multiply by the coefficient: 4 × 3 = 12.
Step 3
12 hydrogen atoms
Worked example 2. How many oxygen atoms does 2Mg(NO₃)₂ represent?
Step 1
Inside one formula unit, each NO₃ holds 3 oxygen atoms, and the outer subscript 2 doubles it: 2 × 3 = 6.
Step 2
Multiply by the coefficient: 2 × 6 = 12.
Step 3
12 oxygen atoms
You can now calculate the number of atoms of each element represented by a chemical formula with a coefficient.
Check your understanding
How many hydrogen atoms does 5H₂O represent?
Answer: 10atoms
One H₂O molecule holds 2 hydrogen atoms. Multiply by the coefficient: 5 × 2 = 10. 5H₂O represents 10 hydrogen atoms.
Check your understanding
How many oxygen atoms does 3Fe₂O₃ represent?
Answer: 9atoms
One Fe₂O₃ formula unit holds 3 oxygen atoms. Multiply by the coefficient: 3 × 3 = 9. 3Fe₂O₃ represents 9 oxygen atoms.
Check your understanding
How many oxygen atoms does 2Al(OH)₃ represent?
Answer: 6atoms
Inside one Al(OH)₃, the subscript 3 outside the parentheses makes 3 OH groups — 3 oxygen atoms. Multiply by the coefficient: 2 × 3 = 6. 2Al(OH)₃ represents 6 oxygen atoms.
Lesson 11 of 51 · RXN-011
The law of conservation of mass
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Have You Ever Wondered?
Wonder this:
A campfire log burns down to a scoop of ash weighing far less than the log did. Did most of the log's mass simply vanish?
You've already seen that mass is conserved in changes. The idea now gets its exact, weighable form as a law.
The idea
The 'law of conservation of mass' says: in a chemical reaction, the total mass of the products equals the total mass of the reactants.
No mass appears, and no mass disappears.
When 4 g of hydrogen reacts completely with 32 g of oxygen, exactly 36 g of water forms.
Reactant side: 4 + 32 = 36 g. Product side: 36 g. The totals match exactly.
The totals match only when every substance is counted — gases included.
So none of the burnt log's mass vanished — every gram is in some substance, counted or not.
Worked examples
Worked example 1. 56 g of iron reacts completely with 32 g of sulfur, forming only iron sulfide. What does the law of conservation of mass say the iron sulfide weighs?
Step 1
Answer: exactly 88 g — the total mass of the products equals the total mass of the reactants.
Worked example 2. State the law of conservation of mass.
Step 1
Answer: in a chemical reaction, the total mass of the products equals the total mass of the reactants.
You can now state the law of conservation of mass: in a chemical reaction, the total mass of the products equals the total mass of the reactants.
Check your understanding
Which statement is the law of conservation of mass, stated for a chemical reaction?
AIn a chemical reaction, the total mass of the products equals the total mass of the reactants.correct
BIn a chemical reaction, the mass of each product equals the mass of each reactant.
This option is wrong — you matched substances one-to-one — the law compares the TOTALS of each side, not individual substances.
CIn a chemical reaction, the products weigh less, because some mass leaves as gas.
This option is wrong — you shrank the product total — a gas product still has mass and counts fully, so the two totals stay equal.
DIn a chemical reaction, mass is conserved only when no gas is involved.
This option is wrong — you carved out gases — gases have mass and count fully; the law holds for every reaction.
The law compares the two sides' totals. Total mass of products = total mass of reactants. No mass appears, and no mass disappears.
Check your understanding
In a sealed flask, 6.4 g of sulfur burns completely in 6.4 g of oxygen, forming only sulfur dioxide. What is the total mass of sulfur dioxide formed?
AExactly 12.8 gcorrect
BLess than 12.8 g
This option is wrong — you let burning shed mass — the oxygen's 6.4 g is inside the sulfur dioxide, and the totals stay equal.
CMore than 12.8 g
This option is wrong — you let the reaction create mass — no mass appears; the product total equals the reactant total.
DIt cannot be told from the masses given
This option is wrong — you set the law aside — the law fixes the product total at exactly the reactant total, 6.4 + 6.4 g.
Total mass of products = total mass of reactants. Reactant total: 6.4 + 6.4 = 12.8 g. So exactly 12.8 g of sulfur dioxide forms.
Check your understanding
Two solutions are sealed together in a flask and react, forming a solid inside. How does the sealed flask's total mass change?
AIt does not change.correct
BIt increases, because solids weigh more than liquids.
This option is wrong — you weighed the state instead of the matter — forming a solid rearranges the same matter, and the total is unchanged.
CIt decreases, because the solid settles to the bottom.
This option is wrong — you let position change mass — where the solid sits has no effect on the flask's total.
DIt depends on which reaction happened.
This option is wrong — you made the law optional — the law holds for every chemical reaction.
The law of conservation of mass holds for every reaction. Total mass of products = total mass of reactants. Sealed in, the flask's total mass cannot change.
Lesson 12 of 51 · RXN-012
Why mass is conserved
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Did You Know?
The law says the totals always match. You've already seen what atoms do in a reaction — and that is the whole reason the law holds.
The idea
Mass is conserved in a chemical reaction because the atoms themselves are not created, destroyed, or changed — they only separate and regroup into new combinations.
Each atom keeps its own mass through the regrouping.
Same atoms, same total mass — regrouping cannot change what everything weighs.
Burn methane in a sealed container: every carbon, hydrogen, and oxygen atom is still there afterward, regrouped into carbon dioxide and water.
The particle diagram shows it — count each kind of atom on the two sides.
1 carbon, 4 hydrogens, 4 oxygens on each side — same atoms, same total mass.
Worked examples
Worked example 1. A sealed jar holds damp steel wool. Over days the iron rusts. Why does the jar's reading on the balance stay the same?
Step 1
Every atom in the jar before the rusting is still in the jar after it.
Step 2
The atoms only regrouped into rust, and each atom kept its mass — so the total mass is unchanged.
Worked example 2. A candle is lit inside a sealed jar and burns until the flame dies. Why is the jar's total mass the same before and after?
Step 1
The wax and oxygen atoms were not destroyed by the flame.
Step 2
They regrouped into carbon dioxide and water, each atom keeping its mass, so the sealed jar's total is unchanged.
You can now explain that mass is conserved in a chemical reaction because the atoms themselves are not created, destroyed, or changed — they only separate and regroup into new combinations, and each atom keeps its mass.
Check your understanding
A sealed tube of copper turnings is heated until the copper reacts with the oxygen sealed inside, forming black copper(II) oxide. Why is the tube's total mass unchanged?
AThe copper and oxygen atoms only regrouped into copper(II) oxide, and each atom kept its mass.correct
BThe heating added exactly enough mass to replace the oxygen that was used.
This option is wrong — you gave heat a mass — heating adds no matter, and the oxygen was never lost, only regrouped.
CNew atoms formed inside the solid to replace the gas atoms.
This option is wrong — you created atoms — no atoms form; the oxygen atoms themselves are now inside the solid.
DThe oxygen atoms were destroyed, but their mass stayed behind in the tube.
This option is wrong — you destroyed atoms and kept their mass — atoms are not destroyed, and mass belongs to the atoms that carry it.
The atoms themselves are not created, destroyed, or changed — they only separate and regroup into new combinations. Each atom keeps its own mass. Same atoms, same total mass — the sealed tube weighs the same.
Check your understanding
Why can a chemical reaction never change the total mass, whatever the reaction is?
AThe atoms only regroup — none are created, destroyed, or changed, and each keeps its mass.correct
BThe heat given off always balances the mass taken in.
This option is wrong — you traded heat for mass — heat carries no matter, and no mass is taken in or given off to balance.
CReactions destroy and create atoms in exactly equal numbers.
This option is wrong — you balanced creation against destruction — a reaction does neither; the same atoms persist throughout.
DThe products always contain exactly the same substances as the reactants did.
This option is wrong — you froze the substances — the substances DO change; it is the atoms inside them that persist.
The atoms themselves are not created, destroyed, or changed — they only separate and regroup into new combinations. Each atom keeps its own mass. The same atoms at the same masses give the same total, every time.
Check your understanding
The diagram shows carbon monoxide reacting with oxygen. Count the atoms in each panel. Why must the products weigh exactly what the reactants weighed?
ABoth panels hold the same 2 carbons and 4 oxygens — the same atoms, each keeping its mass.correct
BThe after panel gained an atom to make the heavier carbon dioxide.
This option is wrong — you created an atom — count them: 2 carbons and 4 oxygens appear in both panels.
CThe panels weigh the same because both show exactly three molecules.
This option is wrong — you counted molecules instead of atoms — the after panel has two molecules, and mass follows the atoms, not the molecule count.
DThe oxygen molecule's atoms merged into one heavier atom.
This option is wrong — you changed the atoms — each oxygen atom persists unchanged inside a carbon dioxide molecule.
Count each kind of atom: 2 carbons and 4 oxygens before, 2 carbons and 4 oxygens after. The atoms only separated and regrouped into new combinations. Each atom keeps its mass, so the totals are identical.
Lesson 13 of 51 · RXN-013
Calculating with conservation of mass
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Did You Know?
You've already used matching mass totals to find a missing 'before' or 'after' mass. The same routine now runs in reactant-and-product language — weigh everything but one substance, and the law hands you the missing mass.
The equation
Write down the mass of every reactant and every product given in the question.
Write down the law as an equation: total mass of reactants = total mass of products.
Expand each side into the problem's own substances.
Make the unknown mass the subject.
Substitute the known masses, and calculate — the answer carries the same unit as the given masses.
Check the answer: every single substance must weigh less than its own side's total.
Worked examples
Worked example 1. 12.0 g of carbon burns completely in 32.0 g of oxygen, forming only carbon dioxide. What mass of carbon dioxide forms?
Step 1
Write down the values in the question
mass of carbon = 12.0 g
mass of oxygen = 32.0 g
Step 2
Write down the equation
total mass of reactants = total mass of products
Step 3
Write it in this problem's terms
mass of carbon + mass of oxygen = mass of carbon dioxide
Step 4
Make the unknown the subject
mass of carbon dioxide = mass of carbon + mass of oxygen
Step 5
Substitute in the values, and calculate
mass of carbon dioxide = 12.0 + 32.0
mass of carbon dioxide = 44.0 g
Worked example 2. Nitrogen reacts completely with hydrogen, forming 17.0 g of ammonia and nothing else. 14.0 g of nitrogen was used. What mass of hydrogen reacted?
Step 1
Write down the values in the question
mass of ammonia = 17.0 g
mass of nitrogen = 14.0 g
Step 2
Write down the equation
total mass of reactants = total mass of products
Step 3
Write it in this problem's terms
mass of nitrogen + mass of hydrogen = mass of ammonia
Step 4
Make the unknown the subject
mass of hydrogen = mass of ammonia − mass of nitrogen
Step 5
Substitute in the values, and calculate
mass of hydrogen = 17.0 − 14.0
mass of hydrogen = 3.0 g
You can now calculate an unknown reactant or product mass using the law of conservation of mass.
Check your understanding
In a sealed flask, 7.8 g of potassium reacts completely with 7.1 g of chlorine, forming only potassium chloride. What mass of potassium chloride forms? Give your answer in grams to one decimal place.
Answer: 14.9g(tolerance ±0.05)
Write down the values in the question: mass of potassium = 7.8 g mass of chlorine = 7.1 g Write down the equation: total mass of reactants = total mass of products In this problem's substances: mass of potassium + mass of chlorine = mass of potassium chloride Make the mass of potassium chloride the subject: mass of potassium chloride = mass of potassium + mass of chlorine Substitute in the values, and calculate: mass of potassium chloride = 7.8 + 7.1 mass of potassium chloride = 14.9 g
Check your understanding
Heated in a sealed tube, 24.7 g of copper(II) carbonate breaks down completely into 15.9 g of copper(II) oxide and carbon dioxide gas. What mass of carbon dioxide forms? Give your answer in grams to one decimal place.
Answer: 8.8g(tolerance ±0.05)
Write down the values in the question: mass of copper(II) carbonate = 24.7 g mass of copper(II) oxide = 15.9 g Write down the equation: total mass of reactants = total mass of products In this problem's substances: mass of copper(II) carbonate = mass of copper(II) oxide + mass of carbon dioxide Make the mass of carbon dioxide the subject: mass of carbon dioxide = mass of copper(II) carbonate − mass of copper(II) oxide Substitute in the values, and calculate: mass of carbon dioxide = 24.7 − 15.9 mass of carbon dioxide = 8.8 g
Check your understanding
In a sealed vessel, iron reacts completely with chlorine, forming 32.5 g of iron(III) chloride and nothing else. 11.2 g of iron was used. What mass of chlorine reacted? Give your answer in grams to one decimal place.
Answer: 21.3g(tolerance ±0.05)
Write down the values in the question: mass of iron(III) chloride = 32.5 g mass of iron = 11.2 g Write down the equation: total mass of reactants = total mass of products In this problem's substances: mass of iron + mass of chlorine = mass of iron(III) chloride Make the mass of chlorine the subject: mass of chlorine = mass of iron(III) chloride − mass of iron Substitute in the values, and calculate: mass of chlorine = 32.5 − 11.2 mass of chlorine = 21.3 g
Lesson 14 of 51 · RXN-014
Mass changes in open containers
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Have You Ever Wondered?
Wonder this:
Drop baking soda into vinegar in an open beaker on a balance and watch the reading: 205.0 g… 204.1… 202.8. The mixture fizzes, and grams seem to vanish. The law of conservation of mass says that cannot happen. So where did the mass go?
You've already seen the law of conservation of mass, and the state symbol (g) that marks a gas. This lesson explains why an open container seems to break the law — and why it never really does.
The idea
A balance can only weigh what is sitting on it.
In an open container, a gas made by the reaction escapes into the air.
The escaping gas carries its own mass away with it.
So the balance reading falls — not because mass was destroyed, but because part of the mass has left the container.
In the fizzing beaker, the reading fell from 205.0 g to 202.8 g because 2.2 g of carbon dioxide gas escaped.
Count the escaped gas and the law holds exactly: 202.8 g on the balance plus 2.2 g of gas in the air is still 205.0 g.
The balance weighs only what stays in the beaker. The 2.2 g the reading lost is in the escaped gas.
Some reactions do the opposite and take a gas in from the air.
Then the balance reading rises, because gas from the air has become part of what sits on the balance.
A state symbol (g) in the equation tells you which way to expect the reading to move: a gas product escapes, a gas reactant taken from the air is pulled in.
Worked examples
Worked example 1. A student heats 12.4 g of green copper carbonate powder in an open test tube: CuCO₃(s) → CuO(s) + CO₂(g). The powder turns black. Will the test tube's contents weigh more than, less than, or the same as 12.4 g afterwards?
Step 1
The equation shows a gas product: CO₂(g).
Step 2
The test tube is open, so the carbon dioxide escapes into the air.
Step 3
The escaping gas carries its mass away from the test tube.
Step 4
Less than 12.4 g — the missing mass is in the carbon dioxide gas that escaped.
Worked example 2. An open crucible holding magnesium ribbon is heated. The magnesium burns with a bright light and becomes a white powder, magnesium oxide. The crucible and contents weighed 32.4 g before heating and 34.0 g after. Explain the rise.
Step 1
Burning magnesium takes in oxygen gas from the air.
Step 2
The oxygen atoms become part of the white powder in the crucible.
Step 3
Gas taken in from the air adds its mass to what sits on the balance.
Step 4
The reading rose because 1.6 g of oxygen from the air joined the magnesium — mass that entered, not mass that appeared from nowhere.
You can now explain why the measured mass appears to change when a reaction in an open container produces or takes in a gas.
Check your understanding
Hydrogen peroxide solution slowly breaks down in an open flask on a balance: 2H₂O₂(aq) → 2H₂O(l) + O₂(g). Over a week, the balance reading falls. Why does the reading fall?
AOxygen gas escaped from the open flask, carrying its mass with it.correct
BThe reaction destroyed some of the mixture's mass.
This option is wrong — you let a reaction destroy mass — atoms only regroup, so the missing grams must have left the flask, and the (g) in the equation shows what carried them out.
CLiquid that turns into a gas stops having mass.
This option is wrong — you treated a gas as weightless — the oxygen keeps every gram of its mass; the reading falls because that mass is no longer in the flask.
DA balance reads lower while bubbles are forming in the liquid.
This option is wrong — you blamed the balance — the reading falls because oxygen gas really left the flask, and it stays lower after the bubbling stops.
The flask is open, and the equation shows a gas product: O₂(g). The oxygen gas escapes into the air and carries its mass away. The balance now weighs only what is left in the flask, so the reading falls.
Check your understanding
Damp steel wool sits on an open dish on a balance. Over several days it turns to orange rust, and the balance reading rises. Why does the reading rise?
AOxygen gas from the air joined the iron, adding its mass to the dish.correct
BMass was created when the new substance formed.
This option is wrong — you let a reaction create mass — atoms are never created; the extra grams came into the dish from the air.
CThe iron atoms themselves became heavier when they turned into rust.
This option is wrong — you changed the atoms — the atoms themselves are not created, destroyed, or changed; the gain is whole oxygen atoms joining from the air, not heavier iron atoms.
DRust presses down on the balance pan more strongly than iron does.
This option is wrong — you invented a force — a balance reads mass, and the reading rose because oxygen's mass was added to what the balance weighs.
Rusting takes in oxygen gas from the air. The oxygen atoms become part of the rust on the dish. Gas pulled in from the air adds its mass, so the reading rises.
Check your understanding
Crushed eggshell and lemon juice are sealed inside a strong plastic bottle standing on a balance. The mixture fizzes and the bottle swells. What happens to the balance reading while the bottle stays sealed?
AIt stays the same, because nothing has entered or left the bottle.correct
BIt falls, because a gas formed inside the bottle.
This option is wrong — you treated gas inside a sealed bottle as weightless — the gas is still in the bottle, so its mass still counts in the reading.
CIt rises, because the bottle swelled.
This option is wrong — you weighed size instead of mass — swelling changes the bottle's volume, not the amount of matter inside it.
DIt falls at first and then returns to the starting value.
This option is wrong — you let mass dip during the reaction — at every moment the same atoms are inside the sealed bottle, so the reading never moves.
Sealed means nothing gets in and nothing gets out. Every atom, including the gas, stays inside the bottle. The balance weighs the same matter throughout, so the reading stays the same.
Lesson 15 of 51 · RXN-015
Atom conservation in particle diagrams
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Did You Know?
You've already seen why mass is conserved: the atoms themselves are not created, destroyed, or changed — they only separate and regroup into new combinations. A particle diagram of a reaction has to show exactly that. This lesson checks whether a diagram does.
The idea
A particle diagram of a reaction shows the particles before the reaction and the particles after.
Use the key to tell which circle stands for which kind of atom.
Before: two hydrogen molecules and one oxygen molecule. After: two water molecules. Count each kind of circle — every atom is still there, regrouped.
Count the atoms of one kind in the before panel, then count the same kind in the after panel.
Repeat the count for every kind of atom in the diagram.
Equal counts for every kind, before and after — the diagram conserves atoms.
If any kind of atom appears or disappears, the diagram is wrong, because atoms cannot be created or destroyed.
Count the figure for yourself: 4 hydrogen atoms and 2 oxygen atoms before, and 4 hydrogen atoms and 2 oxygen atoms after — the diagram conserves atoms.
Worked examples
Worked example 1. Does the particle diagram in the figure conserve atoms?
Step 1
Carbon: 1 black circle before, 1 black circle after — equal.
Step 2
Oxygen: 2 red circles before, 2 red circles after — equal.
Step 3
Every kind of atom has equal counts.
Step 4
The diagram conserves atoms.
Worked example 2. Does the particle diagram in the figure conserve atoms?
Step 1
Nitrogen: 2 blue circles before, 1 blue circle after — not equal.
Step 2
Hydrogen: 2 white circles before, 3 white circles after — not equal.
Step 3
Atoms cannot appear or disappear.
Step 4
The diagram does not conserve atoms — it is wrong.
You can now evaluate whether a particle diagram of a chemical reaction conserves atoms, by counting each kind of atom before and after.
Check your understanding
The figure shows a particle diagram of a reaction between hydrogen and chlorine. Count each kind of atom in both panels. Which statement is correct?
AEvery kind of atom is conserved.correct
BHydrogen atoms are conserved, but chlorine atoms are not.
This option is wrong — you may have counted a two-atom chlorine molecule as one chlorine — count circles, not molecules: 2 chlorine circles before and 2 after.
CChlorine atoms are conserved, but hydrogen atoms are not.
This option is wrong — you lost track of the hydrogens inside the product molecules — each HCl holds 1 white circle, so 2 before and 2 after.
DNeither hydrogen atoms nor chlorine atoms are conserved.
This option is wrong — you compared molecule shapes instead of counting atoms — the molecules change, but every circle of each kind is still there.
Hydrogen: 2 white circles before, 2 after — equal. Chlorine: 2 green circles before, 2 after — equal. Every kind of atom has equal counts, so the diagram conserves atoms.
Check your understanding
A textbook draft shows the particle diagram in the figure for sulfur powder reacting with oxygen. Count each kind of atom in both panels. Which statement is correct?
ASulfur atoms are conserved, but oxygen atoms are not.correct
BEvery kind of atom is conserved.
This option is wrong — you missed the extra oxygen — 2 red circles before but 3 after; an oxygen atom cannot appear from nowhere.
COxygen atoms are conserved, but sulfur atoms are not.
This option is wrong — you swapped the counts — sulfur is 1 and 1, while oxygen is 2 before and 3 after.
DNeither sulfur atoms nor oxygen atoms are conserved.
This option is wrong — you marked sulfur wrong too — the single yellow circle appears once in each panel; only oxygen's counts differ.
Sulfur: 1 yellow circle before, 1 after — equal. Oxygen: 2 red circles before, 3 after — not equal. An oxygen atom appeared from nowhere, so the diagram does not conserve atoms.
Check your understanding
A student draws the particle diagram in the figure for magnesium burning in oxygen. Count each kind of atom in both panels. Which statement is correct?
ANeither magnesium atoms nor oxygen atoms are conserved.correct
BEvery kind of atom is conserved.
This option is wrong — you matched the panels by look — count the circles: magnesium is 2 before and 3 after, oxygen is 2 before and 3 after.
CMagnesium atoms are conserved, but oxygen atoms are not.
This option is wrong — you under-counted the product panel — each of the 3 product units holds a gray circle, so magnesium is 2 before and 3 after.
DOxygen atoms are conserved, but magnesium atoms are not.
This option is wrong — you under-counted the product panel — each of the 3 product units holds a red circle, so oxygen is 2 before and 3 after.
Magnesium: 2 gray circles before, 3 after — not equal. Oxygen: 2 red circles before, 3 after — not equal. Atoms of both kinds appeared from nowhere, so the diagram does not conserve atoms. One note on the drawing: each touching gray-and-red pair stands for one formula unit of ionic MgO — the lattice drawn one unit at a time, not a separate molecule.
Lesson 16 of 51 · RXN-016
What balanced means
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Did You Know?
Wonder this:
Iron and oxygen make rust, so a student writes: Fe + O₂ → Fe₂O₃. Count the iron atoms — one on the left, two on the right. An iron atom cannot appear from nowhere; the law of conservation of mass forbids it. Equations need a word for getting this right.
You've already counted the atoms a formula with a coefficient represents. One word names an equation whose counts obey the law — the word is about atom counts, not the weighing balance from the mass lessons.
The idea
A chemical equation is 'balanced' when the number of atoms of each element is the same on both sides of the arrow.
Check the counts in H₂ + Cl₂ → 2HCl.
Hydrogen: 2 atoms on the left, and 2HCl holds 2 hydrogen atoms on the right.
Chlorine: 2 atoms on the left and 2 on the right.
Every element has equal counts on both sides, so H₂ + Cl₂ → 2HCl is balanced.
A balanced equation obeys the law of conservation of mass — it shows every atom surviving the reaction.
Each element's atoms, counted on both sides of the arrow. Equal counts for every element: balanced.
An equation whose counts differ for any element is not balanced, and it cannot show what really happens.
Worked examples
Worked example 1. In 2CO + O₂ → 2CO₂, each side shows 2 carbon atoms and 4 oxygen atoms. What word describes this equation?
Step 1
Equal counts for every element on both sides is the definition.
Step 2
Balanced.
Worked example 2. What must be true of the atom counts in a balanced chemical equation?
Step 1
The definition asks one thing of the counts.
Step 2
The number of atoms of each element is the same on both sides of the arrow.
You can now state that a chemical equation is balanced when the number of atoms of each element is the same on both sides of the arrow.
Check your understanding
What is true of a balanced chemical equation?
AThe number of atoms of each element is the same on both sides of the arrow.correct
BThe number of molecules is the same on both sides of the arrow.
This option is wrong — you counted molecules — molecules can regroup into more or fewer molecules; it is the atoms of each element that must match.
CThe number of different substances is the same on both sides of the arrow.
This option is wrong — you counted substances — two reactants can form one product in a perfectly balanced equation; only atom counts must match.
DThe coefficients add up to the same total on both sides of the arrow.
This option is wrong — you added coefficients — coefficients are tools for matching ATOM counts, and their own totals can differ in a balanced equation.
Balanced is a statement about atoms. A chemical equation is balanced when the number of atoms of each element is the same on both sides of the arrow. Molecule counts, substance counts, and coefficient totals are all allowed to differ.
Check your understanding
In N₂ + 3H₂ → 2NH₃, each side shows 2 nitrogen atoms and 6 hydrogen atoms. What does this make the equation?
ABalanced.correct
BNot balanced.
This option is wrong — you may have compared the molecule counts — 4 molecules on the left, 2 on the right — but balanced is about atoms, and both elements' atom counts match.
CBalanced for nitrogen but not for hydrogen.
This option is wrong — you second-guessed a supplied count — hydrogen is 6 on each side, so both elements match.
DImpossible to say without knowing the masses.
This option is wrong — you reached for masses — the definition of balanced uses atom counts alone, and they are given.
Nitrogen: 2 and 2 — equal. Hydrogen: 6 and 6 — equal. Equal counts for every element on both sides: the equation is balanced.
Check your understanding
Why does a correctly written chemical equation have to be balanced?
ABecause the atoms themselves are not created, destroyed, or changed — they only separate and regroup into new combinations.correct
BBecause a chemical reaction must always end with exactly the same number of molecules that it started with.
This option is wrong — you conserved molecules — molecules break up and regroup freely; it is the atoms inside them that survive unchanged.
CBecause chemists agreed on the matching-counts convention, even though real reactions do not follow it.
This option is wrong — you made balance a mere convention — real reactions genuinely keep every atom, and the equation must show that.
DBecause a gas could escape if the two sides of the equation did not match.
This option is wrong — you mixed in the open-container story — escaping gas explains a falling balance reading, not why the equation's counts must match.
An equation is a picture of what the atoms do. The atoms themselves are not created, destroyed, or changed — they only separate and regroup into new combinations. So the same atoms must appear on both sides: the equation must be balanced.
Lesson 17 of 51 · RXN-017
Judging whether an equation is balanced
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Did You Know?
You've already seen what balanced means, and how to count the atoms a formula with a coefficient represents. This lesson turns the definition into a routine you can run on any equation.
The idea
To judge an equation, count each element's atoms on the left side, then on the right side.
Multiply through coefficients as you count: 2NH₃ holds 2 nitrogen atoms and 6 hydrogen atoms.
Compare the two counts element by element.
Equal counts for every element — the equation is balanced.
A different count for any single element — the equation is not balanced.
Run the routine on N₂ + 2H₂ → 2NH₃: nitrogen is 2 and 2, but hydrogen is 4 on the left and 6 on the right.
One mismatch is enough — N₂ + 2H₂ → 2NH₃ is not balanced.
Nitrogen matches, hydrogen does not. One mismatched element is enough: not balanced.
Worked examples
Worked example 1. Is 2Mg + O₂ → 2MgO balanced?
Step 1
Magnesium: 2 on the left, 2 on the right — equal.
Step 2
Oxygen: 2 on the left, 2 on the right — equal.
Step 3
Every element matches — the equation is balanced.
Worked example 2. Is Fe + O₂ → Fe₂O₃ balanced?
Step 1
Iron: 1 on the left, 2 on the right — not equal.
Step 2
One mismatch is enough to decide.
Step 3
The equation is not balanced.
You can now judge whether a chemical equation is balanced by counting the atoms of each element on both sides.
Check your understanding
Count the atoms in the proposed equation Na + Cl₂ → NaCl. Which element, if any, has unequal counts on the two sides?
AChlorine.correct
BSodium.
This option is wrong — you miscounted sodium — it shows 1 atom on each side; it is chlorine that shows 2 on the left and only 1 on the right.
CBoth sodium and chlorine.
This option is wrong — you marked sodium wrong too — sodium is 1 and 1; only chlorine's counts differ.
DNo element — the equation is balanced.
This option is wrong — you missed chlorine's subscript — Cl₂ holds 2 chlorine atoms, but NaCl holds only 1.
Sodium: 1 on the left, 1 on the right — equal. Chlorine: 2 on the left, 1 on the right — not equal. Chlorine's mismatch means the equation is not balanced.
Check your understanding
Count the atoms in the proposed equation C + O₂ → CO₂. Which element, if any, has unequal counts on the two sides?
ANo element — the equation is balanced.correct
BCarbon.
This option is wrong — you miscounted carbon — 1 atom on the left and 1 inside CO₂ on the right.
COxygen.
This option is wrong — you miscounted oxygen — O₂ holds 2 atoms and CO₂ holds 2 atoms, so oxygen matches.
DBoth carbon and oxygen.
This option is wrong — you may have expected coefficients — an equation can be balanced exactly as written, and this one is: carbon 1 and 1, oxygen 2 and 2.
Carbon: 1 on the left, 1 on the right — equal. Oxygen: 2 on the left, 2 on the right — equal. Every element matches, so the equation is balanced.
Check your understanding
Count the atoms in the proposed equation H₂O₂ → H₂O + O₂. Which element, if any, has unequal counts on the two sides?
AOxygen.correct
BHydrogen.
This option is wrong — you miscounted hydrogen — H₂O₂ holds 2 and H₂O holds 2, so hydrogen matches; oxygen shows 2 on the left but 3 on the right.
CBoth hydrogen and oxygen.
This option is wrong — you marked hydrogen wrong too — hydrogen is 2 and 2; only oxygen's counts differ.
DNo element — the equation is balanced.
This option is wrong — you missed the right side's oxygen total — 1 in H₂O plus 2 in O₂ makes 3, against 2 on the left.
Hydrogen: 2 on the left, 2 on the right — equal. Oxygen: 2 on the left, but 1 + 2 = 3 on the right — not equal. Oxygen's mismatch means the equation is not balanced.
Lesson 18 of 51 · RXN-018
From particle diagram to equation
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You've already judged equations by counting atoms, and you've read particle diagrams since Unit 1. A particle diagram and a balanced equation are two pictures of the same reaction — this lesson reads the diagram and finds its equation.
The idea
Read one panel at a time: before-panel molecules become the left side of the equation, after-panel molecules become the right side.
Touching circles form one unit — a molecule, or one formula unit of an ionic compound — and the circles it contains give its formula.
Count how many identical copies of a molecule the panel shows — that count is the formula's coefficient.
A coefficient of 1 is never written.
In the figure, the before panel shows 2 molecules of H₂ and 1 molecule of O₂, and the after panel shows 2 molecules of H₂O.
So the diagram represents 2H₂ + O₂ → 2H₂O.
Molecule counts become coefficients: 2 H₂, 1 O₂, 2 H₂O reads as 2H₂ + O₂ → 2H₂O.
Check the equation the usual way: 4 hydrogen atoms and 2 oxygen atoms on each side.
Worked examples
Worked example 1. Which equation does the particle diagram in the figure represent?
Step 1
Before panel: 2 molecules of CO and 1 molecule of O₂.
Step 2
After panel: 2 molecules of CO₂.
Step 3
Molecule counts become coefficients.
Step 4
2CO + O₂ → 2CO₂
Worked example 2. Which equation does the particle diagram in the figure represent?
Step 1
Before panel: 1 molecule of N₂ and 1 molecule of O₂.
Step 2
After panel: 2 molecules of NO.
Step 3
Coefficients of 1 are never written.
Step 4
N₂ + O₂ → 2NO
You can now identify the balanced chemical equation that a particle diagram represents.
Check your understanding
Which equation does the particle diagram in the figure represent?
AC + O₂ → CO₂correct
BC + O₂ → CO
This option is wrong — you dropped an oxygen — the product molecule in the diagram holds one black circle and TWO red circles, so it is CO₂.
CC + 2O → CO₂
This option is wrong — you split the oxygen molecule into separate atoms — the two red circles touch, so they are one O₂ molecule, not 2O.
DCO₂ → C + O₂
This option is wrong — you read the panels backwards — the before panel becomes the left side of the equation, and the arrow points from before to after.
Before panel: 1 carbon atom alone and 1 molecule of O₂. After panel: 1 molecule of CO₂. Coefficients of 1 are never written: C + O₂ → CO₂.
Check your understanding
Which equation does the particle diagram in the figure represent?
A2Mg + O₂ → 2MgOcorrect
BMg₂ + O₂ → Mg₂O₂
This option is wrong — you blobbed the copies into one formula — Mg₂ and Mg₂O₂ are not real substances; two separate gray circles are 2Mg, a coefficient, not a subscript on one molecule.
CMg + O₂ → MgO
This option is wrong — you ignored the molecule counts — the panels show 2 magnesium atoms and 2 product units, and those counts are the coefficients.
D2MgO → 2Mg + O₂
This option is wrong — you read the panels backwards — the before panel becomes the left side of the equation.
Before panel: 2 separate magnesium atoms and 1 molecule of O₂. After panel: 2 units of MgO. Molecule counts become coefficients: 2Mg + O₂ → 2MgO.
Check your understanding
Which equation does the particle diagram in the figure represent?
AN₂ + 3H₂ → 2NH₃correct
BN₂ + H₂ → NH₃
This option is wrong — you ignored the molecule counts — the before panel shows THREE hydrogen molecules and the after panel shows TWO ammonia molecules.
CN₂ + 3H₂ → N₂H₆
This option is wrong — you blobbed the two ammonia molecules into one formula — N₂H₆ is not a substance at all; two separate molecules are written 2NH₃, a coefficient, not doubled subscripts.
D2NH₃ → N₂ + 3H₂
This option is wrong — you read the panels backwards — the before panel becomes the left side of the equation.
Before panel: 1 molecule of N₂ and 3 molecules of H₂. After panel: 2 molecules of NH₃. Molecule counts become coefficients: N₂ + 3H₂ → 2NH₃.
Lesson 19 of 51 · RXN-019
From equation to particle diagram
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You've already read a particle diagram into its equation. Drawing runs the same rules in reverse — the equation tells you exactly what to draw.
The idea
Draw two panels joined by an arrow: reactants in the before panel, products in the after panel.
Each formula's coefficient tells you how many copies of that molecule to draw.
Each formula's subscripts tell you how many atoms of each kind to draw inside one copy.
Draw the atoms of one molecule touching, and leave clear space between separate molecules.
Use one color of circle per element, and give the diagram a key.
For N₂ + 3H₂ → 2NH₃, the before panel needs 1 molecule of N₂ and 3 molecules of H₂, and the after panel needs 2 molecules of NH₃.
Finish with the conservation check: 2 nitrogen and 6 hydrogen atoms in each panel.
Coefficients set the molecule counts; subscripts set the atoms inside each molecule. Check: 2 nitrogen and 6 hydrogen atoms in each panel.
Worked examples
Worked example 1. Draw the particle diagram for H₂ + Cl₂ → 2HCl.
Step 1
Before panel: no coefficients, so 1 molecule of H₂ and 1 molecule of Cl₂.
Step 2
After panel: the coefficient 2, so 2 molecules of HCl, each one hydrogen circle touching one chlorine circle.
Step 3
Conservation check: 2 hydrogen and 2 chlorine atoms in each panel.
Step 4
The figure shows the finished drawing.
Worked example 2. Draw the particle diagram for 2Mg + O₂ → 2MgO.
Step 1
Before panel: the coefficient 2 means 2 separate magnesium atoms, and O₂ is 1 molecule of two touching oxygen circles.
Step 2
After panel: 2 units of MgO, each one magnesium circle touching one oxygen circle.
Step 3
Conservation check: 2 magnesium and 2 oxygen atoms in each panel.
Step 4
The figure shows the finished drawing.
You can now draw a particle diagram that represents a given balanced chemical equation, showing every atom conserved.
Your turn
On paper, draw the particle diagram for 2H₂ + O₂ → 2H₂O. Draw a before panel and an after panel joined by an arrow, and include a key. When your drawing is finished, reveal the model answer and check your drawing against the checklist.
Model answer. Before panel: 2 molecules of H₂ (each two small white circles touching) and 1 molecule of O₂ (two red circles touching), with clear gaps between molecules. After panel: 2 molecules of H₂O (each one red circle with two small white circles touching it). Key: white = hydrogen, red = oxygen.
The before panel shows exactly 2 two-atom hydrogen molecules and 1 two-atom oxygen molecule.
The after panel shows exactly 2 water molecules, each with 1 oxygen circle and 2 hydrogen circles touching.
Atoms touch only within a molecule, with clear gaps between separate molecules.
Each element has its own circle color, matching a key.
The count check works: 4 hydrogen and 2 oxygen circles in each panel.
Check your understanding
Which particle diagram represents C + O₂ → CO₂?
Acorrect
B
This option is wrong — you split the oxygen molecule into two separate atoms in the before panel — O₂ is drawn as two red circles touching.
C
This option is wrong — you dropped an oxygen from the product — CO₂ needs two red circles touching the black circle, and your after panel loses an atom.
D
This option is wrong — you added an extra O₂ molecule to the after panel — the equation's product is CO₂ alone, and the extra molecule creates atoms from nowhere.
The equation asks for a before panel of 1 lone carbon circle plus 1 two-atom oxygen molecule, and an after panel of exactly 1 CO₂ molecule — a black circle with a red circle touching each side. Count check: 1 carbon and 2 oxygen circles in each panel.
Your turn
On paper, draw the particle diagram for S + O₂ → SO₂. Draw a before panel and an after panel joined by an arrow, and include a key. When your drawing is finished, reveal the model answer and check your drawing against the checklist.
Model answer. Before panel: 1 lone sulfur circle and 1 molecule of O₂ (two red circles touching), clearly separated. After panel: 1 molecule of SO₂ — one yellow circle with two red circles touching it. Key: yellow = sulfur, red = oxygen.
The before panel shows 1 lone sulfur circle, not touching anything.
The before panel's oxygen is 1 molecule of two touching circles, not two separate atoms.
The after panel shows exactly 1 SO₂ molecule: 1 sulfur circle with 2 oxygen circles touching it.
Each element has its own circle color, matching a key.
The count check works: 1 sulfur and 2 oxygen circles in each panel.
Lesson 20 of 51 · RXN-020
Balancing by inspection
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You've already caught unbalanced equations by counting, and you've seen that changing a subscript turns a formula into a different substance. Now comes the fix.
The idea
'Balancing' an equation means placing coefficients until every element's counts match — the formulas themselves never change.
Step 1 — count: tally each element's atoms on both sides.
Step 2 — adjust: pick one element with unequal counts, and fix it by placing a coefficient in front of a whole formula.
Step 3 — recount: a new coefficient changes every element in that formula, so tally everything again.
Repeat adjust and recount, one element at a time, until every count matches.
Never change a subscript — that turns the formula into a different substance.
Use the smallest whole-number coefficients that work.
Try it on Mg + O₂ → MgO: the counts are magnesium 1 and 1, oxygen 2 and 1.
Oxygen is unequal, so place a 2 in front of MgO.
Recount: oxygen is 2 and 2, but magnesium is now 1 and 2.
Place a 2 in front of Mg, and recount: magnesium 2 and 2, oxygen 2 and 2.
Every element matches: 2Mg + O₂ → 2MgO is balanced.
Count, adjust one element, recount — until every element matches.
Worked examples
Worked example 1. Balance Na + Cl₂ → NaCl.
Step 1
Count: sodium 1 and 1, chlorine 2 and 1.
Step 2
Adjust chlorine: place a 2 in front of NaCl — Na + Cl₂ → 2NaCl.
Step 3
Recount: chlorine 2 and 2, sodium 1 and 2.
Step 4
Adjust sodium: place a 2 in front of Na — 2Na + Cl₂ → 2NaCl.
Step 5
Recount: sodium 2 and 2, chlorine 2 and 2 — every element matches.
Step 6
2Na + Cl₂ → 2NaCl
Worked example 2. Balance Al + O₂ → Al₂O₃.
Step 1
Count: aluminum 1 and 2, oxygen 2 and 3.
Step 2
Adjust oxygen: counts of 2 and 3 meet at 6 — place a 3 in front of O₂ and a 2 in front of Al₂O₃.
Step 3
Recount: oxygen 6 and 6, aluminum 1 and 4.
Step 4
Adjust aluminum: place a 4 in front of Al.
Step 5
Recount: aluminum 4 and 4, oxygen 6 and 6 — every element matches.
Step 6
4Al + 3O₂ → 2Al₂O₃
You can now balance a chemical equation by adjusting coefficients one element at a time, never changing subscripts.
Check your understanding
Balance the equation K + Br₂ → KBr. Enter the balanced equation.
Accepted answer: 2K + Br₂ → 2KBr
Count: potassium 1 and 1, bromine 2 and 1. Adjust bromine with a 2 in front of KBr, recount, then fix potassium with a 2 in front of K. 2K + Br₂ → 2KBr — potassium 2 and 2, bromine 2 and 2.
Check your understanding
Balance the equation N₂ + H₂ → NH₃. Enter the balanced equation.
Accepted answer: N₂ + 3H₂ → 2NH₃
Count: nitrogen 2 and 1, hydrogen 2 and 3. Place a 2 in front of NH₃ for nitrogen, recount, then a 3 in front of H₂ for hydrogen. N₂ + 3H₂ → 2NH₃ — nitrogen 2 and 2, hydrogen 6 and 6.
Check your understanding
To balance H₂ + O₂ → H₂O, a student writes H₂ + O₂ → H₂O₂ and declares the job done. What went wrong?
AThe student changed a subscript, turning water into a different substance — only coefficients may be placed.correct
BNothing went wrong — hydrogen and oxygen now show matching counts on the two sides, so the equation is balanced.
This option is wrong — you accepted matching counts at any price — H₂O₂ is hydrogen peroxide, not water, so the equation now describes the wrong reaction.
CThe student adjusted the wrong substance and should have changed O₂ to O instead.
This option is wrong — you fixed one subscript tampering with another — O is not the substance oxygen gas; formulas never change during balancing.
DThe equation was already balanced, so no work was needed.
This option is wrong — you skipped the count — as first written, oxygen shows 2 atoms on the left and only 1 on the right.
Balancing places coefficients — the formulas themselves never change. H₂O₂ is a different substance from H₂O. The correct moves give 2H₂ + O₂ → 2H₂O.
Lesson 21 of 51 · RXN-021
Balancing with polyatomic ions
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You've already balanced equations by counting single elements, and in Unit 4 you met polyatomic ions such as nitrate, sulfate, carbonate, and hydroxide. Formulas that hold polyatomic ions offer a shortcut.
The idea
Check whether a polyatomic ion appears unchanged on both sides of the equation.
If it does, count the whole ion as a single unit instead of counting its atoms one by one.
Try Ba(NO₃)₂ + Na₂SO₄ → BaSO₄ + NaNO₃: nitrate appears unchanged on both sides, and so does sulfate.
Count in units: barium 1 and 1, nitrate 2 and 1, sodium 2 and 1, sulfate 1 and 1.
Nitrate and sulfate appear unchanged on both sides — count each boxed group as one unit.
Adjust nitrate: place a 2 in front of NaNO₃.
Recount: nitrate 2 and 2, sodium 2 and 2 — every count matches.
The balanced equation is Ba(NO₃)₂ + Na₂SO₄ → BaSO₄ + 2NaNO₃.
The shortcut is valid only while the polyatomic ion appears unchanged on both sides.
If the ion appears on one side only, or changes, count its atoms one by one instead.
Worked examples
Worked example 1. Balance Pb(NO₃)₂ + KI → PbI₂ + KNO₃.
Step 1
Nitrate appears unchanged on both sides — count it as a unit.
Step 2
Count: lead 1 and 1, nitrate 2 and 1, potassium 1 and 1, iodine 1 and 2.
Step 3
Adjust nitrate: place a 2 in front of KNO₃.
Step 4
Recount: nitrate 2 and 2, potassium 1 and 2 — adjust potassium with a 2 in front of KI.
Step 5
Recount: potassium 2 and 2, iodine 2 and 2 — every count matches.
Step 6
Pb(NO₃)₂ + 2KI → PbI₂ + 2KNO₃
Worked example 2. Balance MgCl₂ + NaOH → Mg(OH)₂ + NaCl.
Step 1
Hydroxide appears unchanged on both sides — count it as a unit.
Step 2
Count: magnesium 1 and 1, chlorine 2 and 1, sodium 1 and 1, hydroxide 1 and 2.
Step 3
Adjust hydroxide: place a 2 in front of NaOH.
Step 4
Recount: hydroxide 2 and 2, sodium 2 and 1 — adjust sodium with a 2 in front of NaCl.
Step 5
Recount: sodium 2 and 2, chlorine 2 and 2 — every count matches.
Step 6
MgCl₂ + 2NaOH → Mg(OH)₂ + 2NaCl
You can now balance a chemical equation containing polyatomic ions by counting each polyatomic ion as a single unit when it appears unchanged on both sides.
Check your understanding
Balance the equation CaCl₂ + Na₂CO₃ → CaCO₃ + NaCl. Enter the balanced equation.
Accepted answer: CaCl₂ + Na₂CO₃ → CaCO₃ + 2NaCl
Carbonate appears unchanged on both sides — count it as a unit. Count: calcium 1 and 1, chlorine 2 and 1, sodium 2 and 1, carbonate 1 and 1. A 2 in front of NaCl fixes sodium and chlorine together: CaCl₂ + Na₂CO₃ → CaCO₃ + 2NaCl.
Check your understanding
Balance the equation Ca(OH)₂ + Na₂CO₃ → CaCO₃ + NaOH. Enter the balanced equation.
Accepted answer: Ca(OH)₂ + Na₂CO₃ → CaCO₃ + 2NaOH
Hydroxide and carbonate both appear unchanged — count each as a unit. Count: calcium 1 and 1, hydroxide 2 and 1, sodium 2 and 1, carbonate 1 and 1. A 2 in front of NaOH fixes hydroxide and sodium together: Ca(OH)₂ + Na₂CO₃ → CaCO₃ + 2NaOH.
Check your understanding
While balancing H₂SO₄ + KOH → K₂SO₄ + H₂O, which group can safely be counted as a single unit?
ASulfate only.correct
BHydroxide only.
This option is wrong — you tracked hydroxide onto the right side — its atoms end up inside H₂O, a changed form, so hydroxide must be counted atom by atom here.
CBoth sulfate and hydroxide.
This option is wrong — you applied the shortcut to a changed ion — sulfate survives intact, but hydroxide's atoms are rearranged into water.
DNeither group.
This option is wrong — you dropped a valid shortcut — SO₄ appears unchanged inside H₂SO₄ and K₂SO₄, so it counts as one unit.
The shortcut is valid only while the ion appears unchanged on both sides. SO₄ sits intact in H₂SO₄ and in K₂SO₄ — one countable unit. OH's atoms end up inside H₂O, so hydrogen and oxygen there are counted one by one.
Lesson 22 of 51 · RXN-022
Balancing burning reactions, oxygen last
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You've already balanced equations one element at a time. Equations for a compound of carbon and hydrogen burning in oxygen gas have a repeating shape — and a fixed order that tames them.
The idea
The unbalanced equation always has the shape: carbon-hydrogen compound + O₂ → CO₂ + H₂O.
Balance carbon first: the compound's carbon subscript becomes the CO₂ coefficient.
Balance hydrogen next: half the compound's hydrogen subscript becomes the H₂O coefficient.
Balance oxygen last, by setting the O₂ coefficient to whatever the right side now needs.
Oxygen can wait until last because O₂ stands alone — changing its coefficient disturbs no other element.
If the right side's oxygen count comes out odd, no whole O₂ coefficient fits — double every coefficient, then set O₂.
Run it on C₃H₈ + O₂ → CO₂ + H₂O: carbon is 3, so place a 3 before CO₂.
Hydrogen is 8, so place a 4 before H₂O.
The right side now holds 6 + 4 = 10 oxygen atoms, so place a 5 before O₂.
Check: C₃H₈ + 5O₂ → 3CO₂ + 4H₂O — carbon 3 and 3, hydrogen 8 and 8, oxygen 10 and 10.
Carbon, then hydrogen, then oxygen last — O₂ stands alone, so its coefficient is set freely at the end.
Worked examples
Worked example 1. Balance CH₄ + O₂ → CO₂ + H₂O.
Step 1
Carbon first: the subscript is 1, so CO₂ keeps no coefficient.
Step 2
Hydrogen next: the subscript is 4, so place a 2 before H₂O.
Step 3
Oxygen last: the right side holds 2 + 2 = 4 oxygen atoms, so place a 2 before O₂.
Step 4
Check: carbon 1 and 1, hydrogen 4 and 4, oxygen 4 and 4.
Step 5
CH₄ + 2O₂ → CO₂ + 2H₂O
Worked example 2. Balance C₂H₆ + O₂ → CO₂ + H₂O.
Step 1
Carbon first: place a 2 before CO₂; hydrogen next: 6 halves to 3 before H₂O.
Step 2
Oxygen count on the right: 4 + 3 = 7 — an odd number, so no whole O₂ coefficient fits.
Step 3
Double every coefficient: 2C₂H₆ + O₂ → 4CO₂ + 6H₂O.
Step 4
Oxygen last: the right side now holds 8 + 6 = 14, so place a 7 before O₂.
Step 5
Check: carbon 4 and 4, hydrogen 12 and 12, oxygen 14 and 14.
Step 6
2C₂H₆ + 7O₂ → 4CO₂ + 6H₂O
You can now balance an equation in which a compound of carbon and hydrogen burns in oxygen by balancing carbon first, then hydrogen, then oxygen last, doubling every coefficient if the oxygen count comes out odd.
Check your understanding
Ethene burns in oxygen. Balance the equation C₂H₄ + O₂ → CO₂ + H₂O. Enter the balanced equation.
Accepted answer: C₂H₄ + 3O₂ → 2CO₂ + 2H₂O
Carbon: subscript 2, so 2CO₂. Hydrogen: half of 4, so 2H₂O. Oxygen last: the right side holds 4 + 2 = 6, so 3O₂. Check: carbon 2 and 2, hydrogen 4 and 4, oxygen 6 and 6.
Check your understanding
Pentane burns in oxygen. Balance the equation C₅H₁₂ + O₂ → CO₂ + H₂O. Enter the balanced equation.
Accepted answer: C₅H₁₂ + 8O₂ → 5CO₂ + 6H₂O
Carbon: subscript 5, so 5CO₂. Hydrogen: half of 12, so 6H₂O. Oxygen last: the right side holds 10 + 6 = 16, so 8O₂. Check: carbon 5 and 5, hydrogen 12 and 12, oxygen 16 and 16.
Check your understanding
In the carbon-first, hydrogen-next order, why is oxygen balanced last?
ABecause O₂ stands alone as an element, its coefficient can be set freely at the end without disturbing carbon or hydrogen.correct
BBecause oxygen appears in only one of the products.
This option is wrong — you misplaced the oxygens — oxygen sits in BOTH products, which is exactly why its total is easiest to read off at the end.
CBecause the carbon and hydrogen counts change whenever the O₂ coefficient changes, so O₂ must be settled after them.
This option is wrong — you inverted the reason — O₂ contains no carbon or hydrogen, so changing its coefficient disturbs nothing else; that independence is why it can wait.
DBecause oxygen is the heaviest element in the equation.
This option is wrong — you reached for mass — balancing order follows the equation's structure, and O₂'s structural feature is standing alone as an element.
O₂ is a lone element on the left — no other element rides on its coefficient. So carbon and hydrogen are settled first, and the right side's oxygen total is read off. The O₂ coefficient is then set freely to match that total.
Lesson 23 of 51 · RXN-023
Write and balance from words
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Did You Know?
You've already translated word equations into formulas, and you've balanced equations three ways — one element at a time, polyatomic ions as units, and burning reactions oxygen-last. You've also seen that seven elements exist as two-atom molecules: H₂, N₂, O₂, F₂, Cl₂, Br₂, I₂. This lesson chains it all: from a sentence to a balanced equation.
The idea
Stage 1 — translate: write the correct formula for every substance named, exactly as Unit 4 and Unit 5 taught.
A formula's subscripts are fixed by the substance itself — they are never adjusted to make balancing easier.
Stage 2 — balance: place coefficients by inspection until every element's counts match.
Pick the fitting routine: polyatomic ions as units when an ion appears unchanged on both sides, oxygen last when a carbon-hydrogen compound burns.
Try it: 'aluminum metal reacts with oxygen gas to form solid aluminum oxide'.
Translate: aluminum is Al, oxygen gas is O₂, and aluminum oxide — from Al³⁺ and O²⁻ — is Al₂O₃.
The skeleton is Al + O₂ → Al₂O₃.
Balance oxygen: counts of 2 and 3 meet at 6, so place a 3 before O₂ and a 2 before Al₂O₃.
Balance aluminum: place a 4 before Al.
Check: 4Al + 3O₂ → 2Al₂O₃ — aluminum 4 and 4, oxygen 6 and 6.
From sentence to balanced equation
stage
what you write
the sentence
aluminum metal reacts with oxygen gas to form solid aluminum oxide
1 — translate
Al, O₂, Al₂O₃
skeleton
Al + O₂ → Al₂O₃
2 — balance
4Al + 3O₂ → 2Al₂O₃
Stage 1 fixes the formulas from the names; stage 2 places the coefficients. Subscripts never move.
Worked examples
Worked example 1. Write and balance the equation for 'sodium metal reacts with chlorine gas to form sodium chloride'.
Step 1
Translate: sodium is Na, chlorine gas is Cl₂, and sodium chloride — from Na⁺ and Cl⁻ — is NaCl.
Step 2
Skeleton: Na + Cl₂ → NaCl.
Step 3
Balance chlorine: place a 2 before NaCl; recount — sodium is 1 and 2.
Step 4
Balance sodium: place a 2 before Na.
Step 5
Check: 2Na + Cl₂ → 2NaCl — sodium 2 and 2, chlorine 2 and 2.
Step 6
2Na + Cl₂ → 2NaCl
Worked example 2. Write and balance the equation for 'iron(III) chloride solution reacts with sodium hydroxide solution to form solid iron(III) hydroxide and sodium chloride solution'.
Step 1
Translate: iron(III) chloride is FeCl₃, sodium hydroxide is NaOH, iron(III) hydroxide is Fe(OH)₃, sodium chloride is NaCl.
Step 2
Skeleton: FeCl₃ + NaOH → Fe(OH)₃ + NaCl.
Step 3
Hydroxide appears unchanged on both sides — count it as a unit: hydroxide is 1 and 3.
Step 4
Balance hydroxide: place a 3 before NaOH; recount — sodium is 3 and 1, so place a 3 before NaCl.
Step 5
Check: FeCl₃ + 3NaOH → Fe(OH)₃ + 3NaCl — iron 1 and 1, chlorine 3 and 3, sodium 3 and 3, hydroxide 3 and 3.
Step 6
FeCl₃ + 3NaOH → Fe(OH)₃ + 3NaCl
You can now write and balance the formula equation for a reaction described in words.
Check your understanding
Write and balance the formula equation: magnesium metal reacts with nitrogen gas to form solid magnesium nitride. Enter the balanced equation.
Accepted answer: 3Mg + N₂ → Mg₃N₂
Translate: Mg, N₂, and — from Mg²⁺ and N³⁻ — Mg₃N₂. Skeleton: Mg + N₂ → Mg₃N₂; nitrogen already matches at 2 and 2. Place a 3 before Mg: 3Mg + N₂ → Mg₃N₂ — magnesium 3 and 3, nitrogen 2 and 2.
Check your understanding
Write and balance the formula equation: the fuel C₆H₁₂ burns in oxygen gas to form carbon dioxide and water. Enter the balanced equation.
Accepted answer: C₆H₁₂ + 9O₂ → 6CO₂ + 6H₂O
Translate: C₆H₁₂ is given, oxygen gas is O₂, carbon dioxide is CO₂, water is H₂O. Carbon first: 6CO₂. Hydrogen next: half of 12 gives 6H₂O. Oxygen last: 12 + 6 = 18 on the right, so 9O₂ — C₆H₁₂ + 9O₂ → 6CO₂ + 6H₂O.
Check your understanding
Write and balance the formula equation: lithium metal reacts with oxygen gas to form solid lithium oxide. Enter the balanced equation.
Accepted answer: 4Li + O₂ → 2Li₂O
Translate: Li, O₂, and — from Li⁺ and O²⁻ — Li₂O. Skeleton: Li + O₂ → Li₂O; oxygen needs a 2 before Li₂O. Recount lithium: 4 on the right, so 4Li — 4Li + O₂ → 2Li₂O.
Summary video — Chemical equations, conservation of mass, and balancing
Chemists publish thousands of newly studied reactions every year. Nobody memorizes them one by one. Instead, almost every reaction falls into one of a handful of repeating patterns — spot the pattern, and an unfamiliar equation stops being a stranger. Here is the first pattern.
You've already read, written, and balanced equations. Now equations get sorted by their shape.
The idea
In 2Mg + O₂ → 2MgO, two reactants form a single product.
A reaction in which two or more reactants combine into a single product is a 'synthesis' reaction.
H₂ + Cl₂ → 2HCl fits the pattern too: two reactants, one product.
The reactants do not have to be elements — the single product is what makes it synthesis.
Compare 2H₂O → 2H₂ + O₂: one reactant splits into two products, so it is not synthesis.
A coefficient does not add products: 2MgO is two units of ONE product, magnesium oxide.
The synthesis pattern
equation
reactants
products
synthesis?
2Mg + O₂ → 2MgO
2
1
✓
H₂ + Cl₂ → 2HCl
2
1
✓
2H₂O → 2H₂ + O₂
1
2
✗
Count substances, not coefficients: two or more reactants and exactly one product make synthesis.
To classify, count the substances on each side: two or more in, exactly one out — synthesis.
Worked examples
Worked example 1. Classify the reaction 2Na + Cl₂ → 2NaCl.
Step 1
Reactants: two — sodium and chlorine.
Step 2
Products: one — sodium chloride.
Step 3
Two or more reactants combine into a single product.
Step 4
Synthesis.
Worked example 2. Classify the reaction CaO + H₂O → Ca(OH)₂.
Step 1
Reactants: two, and both are compounds.
Step 2
Products: one — calcium hydroxide.
Step 3
Reactants being compounds changes nothing — the single product is the tell.
Step 4
Synthesis.
You can now classify a reaction as synthesis when two or more reactants combine into a single product.
Check your understanding
Which equation shows a synthesis reaction?
AS + O₂ → SO₂correct
B2HgO → 2Hg + O₂
This option is wrong — you picked one reactant splitting into two products — synthesis runs the other way: two or more reactants into a single product.
CZn + CuCl₂ → ZnCl₂ + Cu
This option is wrong — you picked an equation with two products — synthesis ends with exactly one.
D2H₂O₂ → 2H₂O + O₂
This option is wrong — you picked one reactant splitting apart — a synthesis equation has two or more reactants and a single product.
Count the substances on each side. S + O₂ → SO₂ has two reactants and exactly one product. Two or more in, one out — synthesis.
Check your understanding
Which equation shows a synthesis reaction?
AN₂ + 3H₂ → 2NH₃correct
B2NH₃ → N₂ + 3H₂
This option is wrong — you picked the same substances running the wrong way — here one reactant splits into two products, the reverse of synthesis.
CCH₄ + 2O₂ → CO₂ + 2H₂O
This option is wrong — you picked an equation with two products — synthesis ends with exactly one.
DFe + CuSO₄ → FeSO₄ + Cu
This option is wrong — you picked an equation with two reactants AND two products — the single product is what synthesis requires.
Count the substances on each side. N₂ + 3H₂ → 2NH₃ has two reactants and one product — the 2 is a coefficient, not a second product. Two in, one out — synthesis.
Check your understanding
A student says 2CO + O₂ → 2CO₂ cannot be synthesis because carbon monoxide is already a compound. Which statement is correct?
AIt is synthesis — two reactants combine into a single product, whether or not they are elements.correct
BThe student is right — synthesis reactions must start from elements.
This option is wrong — you added a rule the pattern does not have — the definition counts substances, not their kinds; compounds can combine into one product.
CIt is not synthesis, because the product contains more than one element.
This option is wrong — you judged the product's makeup — almost every synthesis product contains more than one element; the test is that there is exactly ONE product.
DIt is not synthesis, because 2CO₂ shows two products.
This option is wrong — you read a coefficient as a product count — 2CO₂ is two molecules of ONE product, carbon dioxide.
The definition counts substances: two or more reactants, exactly one product. CO and O₂ are two reactants; CO₂ is the single product. Reactants may be elements or compounds — 2CO + O₂ → 2CO₂ is synthesis.
Lesson 25 of 51 · RXN-025
Predicting synthesis products
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Did You Know?
You've already classified synthesis reactions, and in Unit 4 you predicted a main-group ion's charge from its table position and wrote formulas by criss-crossing. Put together, those skills predict what two reacting elements will make.
The idea
When a main-group metal reacts with a nonmetal, the single product is the ionic compound of their two ions.
Find each element's ion charge from its periodic-table position.
Write the product's formula by criss-crossing the charge numbers, positive ion first.
The criss-cross routine, including reducing matching subscripts, works exactly as it did in Unit 4.
Predict the product of sodium reacting with chlorine: sodium sits in Group 1, so its ion is Na⁺; chlorine sits in Group 17, so its ion is Cl⁻.
Criss-cross 1 and 1: the product is NaCl.
The prediction names the product's formula — writing and balancing the full equation is the routine you already have.
Worked examples
Worked example 1. Predict the formula of the product when calcium reacts with bromine.
Step 1
Charges from position: calcium is Group 2, so Ca²⁺; bromine is Group 17, so Br⁻.
Step 2
Criss-cross: the 2 crosses to Br, the 1 crosses to Ca and goes unwritten.
Step 3
CaBr₂
Worked example 2. Predict the formula of the product when aluminum reacts with sulfur.
Step 1
Charges from position: aluminum is Group 13, so Al³⁺; sulfur is Group 16, so S²⁻.
Step 2
Criss-cross: the 3 crosses to S, the 2 crosses to Al.
Step 3
Al₂S₃
You can now predict the product of a synthesis reaction between two elements, using ion charges to write the product's formula.
Check your understanding
Potassium reacts with iodine. Predict the product and enter its formula.
Accepted answer: KI
Charges from position: K is Group 1, so K⁺; I is Group 17, so I⁻. Criss-cross 1 and 1 — both subscripts go unwritten. The product is KI.
Check your understanding
Magnesium reacts with chlorine. Predict the product and enter its formula.
Accepted answer: MgCl₂
Charges from position: Mg is Group 2, so Mg²⁺; Cl is Group 17, so Cl⁻. Criss-cross: the 2 crosses to Cl, the 1 crosses to Mg and goes unwritten. The product is MgCl₂ — check: (2+) + 2 × (1−) = 0.
Check your understanding
Calcium reacts with sulfur. Which formula is correct for the product?
ACaScorrect
BCa₂S₂
This option is wrong — you criss-crossed without reducing — the 2 and 2 subscripts share a factor, and the smallest whole-number ratio is one of each.
CCaS₂
This option is wrong — you crossed only calcium's charge — sulfur is Group 16, so its ion is S²⁻, and one Ca²⁺ cancels one S²⁻ exactly, giving CaS.
DCa₂S
This option is wrong — you crossed only sulfur's charge — Ca²⁺ and S²⁻ balance one to one, with no subscripts written.
Charges from position: Ca is Group 2, so Ca²⁺; S is Group 16, so S²⁻. Criss-cross gives Ca₂S₂, and reducing the matching subscripts gives one of each. The product is CaS — check: (2+) + (2−) = 0.
Lesson 26 of 51 · RXN-026
Decomposition reactions
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Did You Know?
You've already seen synthesis: two or more reactants, one product. The second pattern is its mirror image.
The idea
In 2H₂O → 2H₂ + O₂, a single compound splits into two simpler substances.
A reaction in which one compound breaks apart into two or more simpler substances is a 'decomposition' reaction.
The tell is the left side: exactly one reactant.
The pieces do not have to be elements — MgCO₃ → MgO + CO₂ is decomposition into two simpler compounds.
Compare CaO + CO₂ → CaCO₃: two reactants forming one product is synthesis, not decomposition.
Most decompositions run only while energy is supplied, by heating the compound or passing electricity through it.
The decomposition pattern
equation
reactants
products
decomposition?
2H₂O → 2H₂ + O₂
1
2
✓
MgCO₃ → MgO + CO₂
1
2
✓
CaO + CO₂ → CaCO₃
2
1
✗
Exactly one reactant splitting into simpler substances makes decomposition; two reactants forming one product is synthesis.
To classify, count the reactants: exactly one, splitting into simpler substances — decomposition.
Worked examples
Worked example 1. Classify the reaction 2HgO → 2Hg + O₂.
Step 1
Reactants: one — mercury(II) oxide.
Step 2
Products: two simpler substances, mercury and oxygen.
Step 3
One compound breaks apart into two or more simpler substances.
Step 4
Decomposition.
Worked example 2. Baking soda breaks down in a hot oven: 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂. Classify the reaction.
Step 1
Reactants: one — sodium hydrogen carbonate.
Step 2
Products: three simpler substances.
Step 3
Two or MORE pieces still fits the pattern; the single reactant is the tell.
Step 4
Decomposition.
You can now classify a reaction as decomposition when a single compound breaks apart into two or more simpler substances.
Check your understanding
Which equation shows a decomposition reaction?
A2NaCl → 2Na + Cl₂correct
B2Na + Cl₂ → 2NaCl
This option is wrong — you picked the same substances running the wrong way — two reactants combining into one product is synthesis.
CZn + 2HCl → ZnCl₂ + H₂
This option is wrong — you picked an equation with two reactants — decomposition starts from exactly one.
DS + O₂ → SO₂
This option is wrong — you picked two reactants forming one product — that is synthesis, the mirror image of decomposition.
Count the reactants. 2NaCl → 2Na + Cl₂ starts from one compound and splits it into two simpler substances. One in, two or more out — decomposition.
Check your understanding
Hydrogen peroxide in a brown bottle slowly turns into water and oxygen: 2H₂O₂ → 2H₂O + O₂. Classify the reaction.
AA decomposition reaction.correct
BA synthesis reaction.
This option is wrong — you read the direction backwards — synthesis combines reactants into ONE product; here one reactant splits into two products.
CBoth a synthesis and a decomposition reaction.
This option is wrong — you let one equation fit both patterns — a reaction cannot have both exactly one product and exactly one reactant splitting; the counts decide.
DNeither a synthesis nor a decomposition reaction.
This option is wrong — you missed the single reactant — one compound breaking into two simpler substances is the decomposition pattern exactly.
Count the reactants: one — hydrogen peroxide. Count the products: two simpler substances. One compound breaking apart — decomposition.
Check your understanding
Green copper carbonate powder turns black when heated: CuCO₃ → CuO + CO₂. Classify the reaction.
AA decomposition reaction.correct
BA synthesis reaction.
This option is wrong — you may have focused on the products being compounds — the pieces need not be elements; one reactant splitting is the tell.
CBoth a synthesis and a decomposition reaction.
This option is wrong — you let one equation fit both patterns — the counts decide, and one reactant with two products fits decomposition only.
DNeither a synthesis nor a decomposition reaction.
This option is wrong — you may have required elements as products — CuO and CO₂ are simpler compounds than CuCO₃, and that still fits decomposition.
Count the reactants: one — copper carbonate. It splits into two simpler compounds, CuO and CO₂. The pieces need not be elements — decomposition.
Lesson 27 of 51 · RXN-027
Predicting decomposition products
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Did You Know?
You've already classified decomposition reactions: a single compound breaks apart into two or more simpler substances. For one family of compounds, you can predict exactly what forms.
The idea
A compound of two elements can break apart into exactly those two elements.
The products are predictable because the compound contains nothing else — atoms of its two elements are all there is.
Heating mercury(II) oxide, HgO, breaks it into mercury and oxygen.
The balanced equation is 2HgO → 2Hg + O₂.
This prediction works only for compounds of two elements.
A compound of three or more elements usually breaks into simpler compounds instead, so you would need more information to predict its products.
Worked examples
Worked example 1. Silver oxide, Ag₂O, breaks down when heated. Predict the products.
Step 1
Ag₂O is a compound of two elements: silver and oxygen.
Step 2
A compound of two elements breaks apart into exactly those two elements.
Step 3
The products are silver and oxygen: 2Ag₂O → 4Ag + O₂.
Worked example 2. An electric current breaks water, H₂O, apart. Predict the products.
Step 1
H₂O is a compound of two elements: hydrogen and oxygen.
Step 2
A compound of two elements breaks apart into exactly those two elements.
Step 3
The products are hydrogen and oxygen: 2H₂O → 2H₂ + O₂.
You can now predict the products of decomposing a compound of two elements into those elements.
Check your understanding
An electric current is passed through molten sodium chloride, NaCl, and the compound breaks down. What products form?
ASodium and chlorine.correct
BSodium and oxygen.
This option is wrong — you brought in an element the compound does not contain — the products can only be the two elements in the formula, sodium and chlorine.
CA new compound of sodium and chlorine.
This option is wrong — you predicted a compound — a compound of two elements breaks apart into its separate elements.
DSodium only.
This option is wrong — you dropped the chlorine — atoms are not destroyed, so every element in the compound must appear in the products.
NaCl is a compound of two elements: sodium and chlorine. A compound of two elements breaks apart into exactly those two elements. The products are sodium and chlorine: 2NaCl → 2Na + Cl₂.
Check your understanding
Molten lead(II) bromide, PbBr₂, is broken down by an electric current. What products form?
ALead and bromine.correct
BLead and oxygen.
This option is wrong — you brought in an element the compound does not contain — PbBr₂ holds only lead and bromine atoms.
CA simpler compound of lead and bromine.
This option is wrong — you predicted a compound — a compound of two elements breaks apart into its separate elements.
DBromine only.
This option is wrong — you dropped the lead — every element in the compound must appear in the products.
PbBr₂ is a compound of two elements: lead and bromine. A compound of two elements breaks apart into exactly those two elements. The products are lead and bromine: PbBr₂ → Pb + Br₂.
Check your understanding
A class knows one decomposition rule: a compound made of two elements breaks apart into those elements. For which compound can they predict the decomposition products from the formula alone?
AKIcorrect
BCaCO₃
This option is wrong — you picked a compound of three elements — it usually breaks into simpler compounds, not its elements, so the products cannot be predicted from the formula alone.
CNaOH
This option is wrong — you picked a compound of three elements — the from-the-formula prediction covers only compounds of two elements.
DKNO₃
This option is wrong — you picked a compound of three elements — the from-the-formula prediction covers only compounds of two elements.
Count the elements in each formula. KI contains only potassium and iodine — two elements, so it breaks apart into exactly those two elements. CaCO₃, NaOH, and KNO₃ each contain three elements, so their products would need more information.
Lesson 28 of 51 · RXN-028
Single replacement reactions
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Wonder this:
Drop a strip of zinc into blue copper(II) sulfate solution. The zinc slowly disappears — and a coat of orange-brown copper grows in its place.
You've classified synthesis and decomposition reactions. The zinc-and-copper change is a third pattern.
The idea
In some reactions, an element on its own pushes another element out of a compound and takes its place.
Zn + CuSO₄ → ZnSO₄ + Cu shows zinc taking copper's place in the compound.
A reaction in which an uncombined element takes the place of an element in a compound is a 'single replacement' reaction.
An uncombined element takes the place of an element in a compound.
The fingerprint is one uncombined element next to one compound on each side of the arrow.
Compare synthesis: 2Na + Cl₂ → 2NaCl also starts with uncombined elements, but the reactants combine into one product — nothing is replaced.
Worked examples
Worked example 1. Classify the reaction Mg + 2HCl → MgCl₂ + H₂.
Step 1
The reactant side shows an uncombined element, Mg, next to a compound, HCl.
Step 2
Magnesium takes hydrogen's place in the compound, and hydrogen leaves as the uncombined element H₂.
Step 3
Single replacement — an uncombined element took the place of an element in a compound.
Worked example 2. Classify the reaction Cu + 2AgNO₃ → Cu(NO₃)₂ + 2Ag.
Step 1
The reactant side shows an uncombined element, Cu, next to a compound, AgNO₃.
Step 2
Copper takes silver's place in the compound, and silver leaves as an uncombined element.
Step 3
Single replacement.
You can now classify a reaction as single replacement when an uncombined element takes the place of an element in a compound.
Check your understanding
Classify the reaction Fe + CuCl₂ → FeCl₂ + Cu.
AA single replacement reactioncorrect
BA synthesis reaction
This option is wrong — you looked for combining — the reactants do not merge into one product; iron takes copper's place in the compound.
CA decomposition reaction
This option is wrong — you looked for breaking apart — nothing splits into simpler substances; an uncombined element replaces an element in a compound.
DNone of these three types
This option is wrong — you missed the fingerprint — one uncombined element next to one compound on each side of the arrow marks single replacement.
Check each side of the arrow: an uncombined element, Fe, sits next to a compound, CuCl₂. Iron takes copper's place, and copper leaves as an uncombined element. An uncombined element taking the place of an element in a compound is single replacement.
Check your understanding
Classify the reaction 2K + Br₂ → 2KBr.
AA synthesis reactioncorrect
BA single replacement reaction
This option is wrong — you saw uncombined elements and jumped to replacement — no compound appears on the reactant side, so there is nothing to replace anything in; the two elements combine into one product.
CA decomposition reaction
This option is wrong — you ran the arrow backwards — the single compound is the product, not the starting substance.
DNone of these three types
This option is wrong — you missed the pattern — two reactants combining into a single product is synthesis.
Both reactants are uncombined elements, and there is only one product. Nothing is replaced — the reactants combine into a single compound. Two or more reactants combining into a single product is synthesis.
Check your understanding
Classify the reaction Al + 3AgNO₃ → Al(NO₃)₃ + 3Ag.
AA single replacement reactioncorrect
BA synthesis reaction
This option is wrong — you looked for combining — there are two products, not one; aluminum takes silver's place in the compound.
CA decomposition reaction
This option is wrong — you looked for breaking apart — no single compound splits; an uncombined element replaces an element in a compound.
DNone of these three types
This option is wrong — you missed the fingerprint — one uncombined element next to one compound on each side of the arrow marks single replacement.
The reactant side shows an uncombined element, Al, next to a compound, AgNO₃. Aluminum takes silver's place, and silver leaves as an uncombined element. That is the single replacement fingerprint.
Lesson 29 of 51 · RXN-029
The activity series
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Did You Know?
Wonder this:
Hold magnesium ribbon in a burner flame and it bursts into blinding white light. An iron nail only glows. A gold ring comes out unchanged, every time. Metals differ enormously in how eagerly they react.
You've just seen single replacement reactions, where one metal pushes another out of a compound. Which metal wins such a contest is not random.
The idea
Chemists compared many metals by observing how readily each one reacts.
The results are collected into a ranked list called an 'activity series'.
An activity series ranks metals from most reactive at the top to least reactive at the bottom.
In the series shown, potassium sits at the top, so potassium is the most reactive metal listed.
Gold sits at the bottom, so gold is the least reactive metal listed.
An activity series ranks metals from most reactive at the top to least reactive at the bottom.
The ranking comes from observed reactions, not from a calculation.
The order exists because a metal atom reacts by giving away its outer electrons, and it reacts more easily when those electrons are easier to remove — the behavior you saw in the periodic table unit, now put in order.
Worked examples
Worked example 1. What does an activity series rank?
Step 1
Answer: metals, from most reactive at the top to least reactive at the bottom.
Worked example 2. Sodium sits near the top of the activity series. What does that position tell you?
Step 1
Answer: sodium is one of the most reactive metals listed.
You can now state that an activity series ranks metals from most reactive to least reactive, based on observed reactions.
Check your understanding
What does an activity series rank?
AMetals, from most reactive to least reactive.correct
BMetals, from heaviest to lightest.
This option is wrong — you ranked by mass — the series ranks how readily metals react, not how much they weigh.
CAll elements, from most metallic to least metallic.
This option is wrong — you stretched the list to every element — this activity series ranks metals only, and by reactivity.
DCompounds, from most stable to least stable.
This option is wrong — you swapped in compounds — the series ranks uncombined metals by how readily they react.
An activity series ranks metals. The ranking runs from most reactive at the top to least reactive at the bottom. It is built from observed reactions.
Check your understanding
A metal sits near the bottom of an activity series. What does that position tell you?
AIt is one of the least reactive metals listed.correct
BIt is one of the most reactive metals listed.
This option is wrong — you flipped the ordering — the most reactive metals sit at the TOP; the bottom holds the least reactive.
CIt is one of the densest metals listed.
This option is wrong — you read the ranking as a density list — the series orders metals by reactivity, not by any physical property.
DIt reacts only when an electric current is applied.
This option is wrong — you invented a condition — a low position just means the metal reacts less readily than the metals above it.
The series runs from most reactive at the top to least reactive at the bottom. A metal near the bottom is therefore one of the least reactive metals listed.
Check your understanding
Where does the ranking in an activity series come from?
AObserved reactions of the metals.correct
BThe metals' atomic masses.
This option is wrong — you used mass — reactivity is about how a metal behaves, and the series records that behavior directly.
CCalculations from the metals' group numbers alone.
This option is wrong — you swapped evidence for theory — the series is built from what metals are actually observed to do, not calculated from the periodic table.
DThe metals' melting points.
This option is wrong — you used a physical property — the series ranks chemical reactivity, which melting points do not measure.
The ranking comes from observed reactions, not from a calculation. Chemists compared many metals by watching how readily each one reacts, and the series records the results.
Lesson 30 of 51 · RXN-030
Reading the activity series
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Did You Know?
You've seen what an activity series is. Reading it answers a direct question: of two metals, which is more reactive?
The idea
Find both metals in the supplied activity series.
The metal that sits higher is the more reactive one.
The metal that sits higher in the series is the more reactive one.
Zinc sits above copper in the series, so zinc is more reactive than copper.
The size of the gap does not matter — higher means more reactive, whether the metals are neighbors or far apart.
Worked examples
Worked example 1. Use the activity series to compare iron and silver. Which is more reactive?
Step 1
Find both metals: iron sits above silver in the series.
Step 2
The metal that sits higher is the more reactive one.
Step 3
Iron is more reactive than silver.
Worked example 2. Use the activity series to compare calcium and potassium. Which is more reactive?
Step 1
Find both metals: potassium sits above calcium in the series.
Step 2
The metal that sits higher is the more reactive one.
Step 3
Potassium is more reactive than calcium — neighbors follow the same higher-means-more-reactive rule.
You can now compare the reactivity of two metals using a supplied activity series.
Check your understanding
Use the activity series shown. Which of these four metals is the most reactive?
AAluminumcorrect
BIron
This option is wrong — you stopped at a familiar metal — iron sits below aluminum in the series, and higher means more reactive.
CCopper
This option is wrong — you picked a metal near the bottom — copper sits below aluminum and iron, so it is less reactive than both.
DSilver
This option is wrong — you picked one of the least reactive metals listed — silver sits near the bottom of the series.
Find all four metals in the series. Aluminum sits highest of the four. The metal that sits higher is the more reactive one, so aluminum is the most reactive of the four.
Check your understanding
Use the activity series shown. Which of these four metals is the least reactive?
ATincorrect
BSodium
This option is wrong — you flipped the ordering — sodium sits near the top, which makes it the MOST reactive of the four.
CMagnesium
This option is wrong — you picked a metal that sits above zinc and tin — lower in the series means less reactive.
DZinc
This option is wrong — you stopped one row too early — tin sits below zinc, so tin is the less reactive of the two.
Find all four metals in the series. Tin sits lowest of the four. Lower in the series means less reactive, so tin is the least reactive of the four.
Check your understanding
Use the activity series shown. Which list orders magnesium, copper, and iron from most reactive to least reactive?
AMagnesium, iron, coppercorrect
BCopper, iron, magnesium
This option is wrong — you read the series upside down — the most reactive metal is the one that sits HIGHEST.
CIron, magnesium, copper
This option is wrong — you swapped the top two — magnesium sits above iron in the series.
DMagnesium, copper, iron
This option is wrong — you swapped the bottom two — iron sits above copper in the series.
Find all three metals in the series. Top to bottom they appear as magnesium, then iron, then copper. Higher means more reactive, so the order is magnesium, iron, copper.
Lesson 31 of 51 · RXN-031
Will single replacement happen?
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Did You Know?
You've classified single replacement reactions and compared metals in the activity series. The series also answers a sharper question: will a single replacement actually happen?
The idea
A metal can replace a metal that sits below it in the activity series.
A metal cannot replace a metal that sits above it.
A metal can replace a metal below it in the series, but not a metal above it.
Copper sits above silver, so copper placed in silver nitrate solution reacts — copper pushes the silver out.
Silver sits below copper, so silver placed in copper(II) nitrate solution does not react at all.
The prediction takes two steps: find both metals in the series, then check whether the uncombined metal sits higher than the metal in the compound.
This series ranks metals only, so it settles metal-replaces-metal cases — whether a metal reacts with an acid is decided a different way, later in your studies.
Worked examples
Worked example 1. A strip of zinc is placed in lead(II) nitrate solution. Use the activity series: does a reaction occur?
Step 1
The uncombined metal is zinc; the metal in the compound is lead.
Step 2
Zinc sits above lead in the series.
Step 3
A metal can replace a metal that sits below it in the activity series.
Step 4
A reaction occurs — zinc replaces the lead.
Worked example 2. An iron nail is placed in magnesium chloride solution. Use the activity series: does a reaction occur?
Step 1
The uncombined metal is iron; the metal in the compound is magnesium.
Step 2
Iron sits below magnesium in the series.
Step 3
A metal cannot replace a metal that sits above it.
Step 4
No reaction occurs.
You can now predict whether a single replacement reaction occurs, using the rule that a metal replaces a metal below it in the activity series but not a metal above it.
Check your understanding
A strip of aluminum is placed in iron(II) chloride solution. Use the activity series shown. Does a reaction occur?
AYes — aluminum sits above iron in the series.correct
BYes — aluminum sits below iron in the series.
This option is wrong — you got the right verdict from the wrong reading — check the series again: aluminum sits ABOVE iron, and that is exactly why it can replace iron.
CNo — aluminum sits above iron in the series.
This option is wrong — you reversed the rule — a metal CAN replace a metal that sits below it, so sitting above iron is what makes the reaction happen.
DNo — aluminum sits below iron in the series.
This option is wrong — you misread the series — aluminum sits above iron, so aluminum can replace it.
The uncombined metal is aluminum; the metal in the compound is iron. Aluminum sits above iron in the series. A metal can replace a metal that sits below it, so a reaction occurs.
Check your understanding
Four beakers are set up. Use the activity series shown. In which beaker does a reaction occur?
AMagnesium metal in tin(II) chloride solution.correct
BCopper metal in zinc sulfate solution.
This option is wrong — you let a lower metal replace a higher one — copper sits below zinc, so copper cannot push zinc out.
CLead metal in magnesium chloride solution.
This option is wrong — you let a lower metal replace a higher one — lead sits far below magnesium in the series.
DSilver metal in iron(II) sulfate solution.
This option is wrong — you let a lower metal replace a higher one — silver sits below iron, so no reaction occurs.
In each beaker, compare the uncombined metal with the metal in the compound. Only magnesium sits above its partner: magnesium is above tin in the series. A metal can replace a metal that sits below it, so only the magnesium beaker reacts.
Check your understanding
Which metal, placed in copper(II) sulfate solution, would NOT react? Use the activity series shown.
ASilvercorrect
BMagnesium
This option is wrong — you picked a metal above copper — magnesium sits well above copper, so it CAN replace copper and would react.
CIron
This option is wrong — you picked a metal above copper — iron sits above copper, so it would react.
DZinc
This option is wrong — you picked a metal above copper — zinc sits above copper, so it would react.
A metal cannot replace a metal that sits above it. Silver sits below copper in the series, so silver cannot push copper out. Magnesium, iron, and zinc all sit above copper, so each of them would react.
Lesson 32 of 51 · RXN-032
Predicting single replacement products
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Did You Know?
You can predict whether a single replacement happens. When it does happen, you can write exactly what forms.
The idea
First check the activity series: the reaction happens only if the uncombined metal sits above the metal in the compound — the series ranks metals only, so the predictions here stay metal-for-metal.
If the uncombined metal sits below, write 'no reaction'.
Step 1 of the routine: the reaction happens only if the uncombined metal sits above the metal in the compound.
If the reaction happens, the incoming metal takes the compound metal's place, and the pushed-out metal is written as an uncombined element.
Write the new compound's formula from the ion charges, exactly as you learned — never copy the old compound's subscripts.
Zinc placed in copper(II) chloride solution: zinc sits above copper, so the reaction happens.
Zinc forms Zn²⁺ ions and chloride is Cl⁻, so the new compound is ZnCl₂.
The full equation is Zn + CuCl₂ → ZnCl₂ + Cu.
Worked examples
Worked example 1. Magnesium is placed in lead(II) nitrate solution. Magnesium forms Mg²⁺ ions and nitrate is NO₃⁻. Predict the products, or write 'no reaction'.
Step 1
Magnesium sits above lead in the series, so the reaction happens.
Step 2
Magnesium takes lead's place, pairing with the nitrate ions.
Step 3
Mg²⁺ and NO₃⁻ give Mg(NO₃)₂.
Step 4
Lead is pushed out as the uncombined element Pb.
Step 5
Mg + Pb(NO₃)₂ → Mg(NO₃)₂ + Pb
Worked example 2. A copper coin is placed in iron(II) sulfate solution. Predict the products, or write 'no reaction'.
Step 1
Copper sits below iron in the series.
Step 2
A metal cannot replace a metal that sits above it.
Step 3
No reaction.
You can now predict the products of a single replacement reaction, or that no reaction occurs, by exchanging the metal and writing correct formulas.
Check your understanding
Aluminum is placed in copper(II) chloride solution and a reaction occurs. Aluminum forms Al³⁺ ions and chloride is Cl⁻. Write the formula of the compound produced.
Accepted answer: AlCl₃
Aluminum takes copper's place, pairing with the chloride ions. Al³⁺ and Cl⁻ give AlCl₃ — the charges set the subscripts, not the old compound. The full equation is 2Al + 3CuCl₂ → 2AlCl₃ + 3Cu.
Check your understanding
Magnesium is placed in silver nitrate solution. Magnesium forms Mg²⁺ ions and nitrate is NO₃⁻. Use the activity series shown. What forms?
AMg(NO₃)₂ and silver metal.correct
BMgNO₃ and silver metal.
This option is wrong — you copied the old compound's one-nitrate pattern — Mg²⁺ needs two NO₃⁻ ions, giving Mg(NO₃)₂.
CNothing — no reaction occurs.
This option is wrong — you failed the series check wrongly — magnesium sits above silver, so magnesium CAN replace it.
DMg(NO₃)₂ and hydrogen gas.
This option is wrong — you pushed out the wrong element — silver is the metal in the compound, so silver is what leaves as an uncombined element.
Magnesium sits above silver, so the reaction happens. Magnesium takes silver's place: Mg²⁺ and NO₃⁻ give Mg(NO₃)₂. Silver is pushed out as the metal: Mg + 2AgNO₃ → Mg(NO₃)₂ + 2Ag.
Check your understanding
A lead strip is placed in zinc sulfate solution. Use the activity series shown. What forms?
ANothing — no reaction occurs.correct
BPbSO₄ and zinc metal.
This option is wrong — you ran the replacement without checking the series — lead sits below zinc, so lead cannot push zinc out.
CPbSO₄ and ZnSO₄.
This option is wrong — you gave both metals the sulfate — there is one sulfate partner, and in any case lead sits below zinc, so no reaction occurs.
DLead and zinc both become uncombined metals.
This option is wrong — you turned a replacement into a decomposition — and the series check fails anyway: lead sits below zinc.
Check the series first: the uncombined metal is lead, and the metal in the compound is zinc. Lead sits below zinc, and a metal cannot replace a metal that sits above it. So the answer is 'no reaction'.
Lesson 33 of 51 · RXN-033
Double replacement reactions
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Did You Know?
You've classified reactions where one uncombined element replaces another. A fourth pattern needs no uncombined element at all: two dissolved compounds trade partners.
The idea
AgNO₃(aq) + NaCl(aq) → AgCl(s) + NaNO₃(aq) starts with two compounds dissolved in water.
The two positive ions trade partners: silver pairs with chloride, and sodium pairs with nitrate.
A reaction in which two compounds dissolved in water exchange partners to form two new compounds is a 'double replacement' reaction.
The two positive ions trade partners — two compounds in, two new compounds out.
The fingerprint is two compounds on each side of the arrow, with the positive ions swapped — and no uncombined element anywhere.
Compare single replacement: Zn + 2AgNO₃ → Zn(NO₃)₂ + 2Ag has an uncombined element on each side, so it is single replacement, not double.
Worked examples
Worked example 1. Classify the reaction Pb(NO₃)₂(aq) + 2KI(aq) → PbI₂(s) + 2KNO₃(aq).
Step 1
Each side of the arrow shows two compounds, and no uncombined element appears.
Step 2
The positive ions traded partners: lead pairs with iodide, and potassium pairs with nitrate.
Step 3
Double replacement — two dissolved compounds exchanged partners to form two new compounds.
Worked example 2. Classify the reaction CuSO₄(aq) + 2NaOH(aq) → Cu(OH)₂(s) + Na₂SO₄(aq).
Step 1
Two compounds appear on each side, with no uncombined element.
Step 2
The positive ions traded partners: copper pairs with hydroxide, and sodium pairs with sulfate.
Step 3
Double replacement.
You can now classify a reaction as double replacement when two compounds dissolved in water exchange partners to form two new compounds.
Check your understanding
Classify the reaction K₂CO₃(aq) + CaCl₂(aq) → CaCO₃(s) + 2KCl(aq).
AA double replacement reactioncorrect
BA single replacement reaction
This option is wrong — you looked for an uncombined element — both reactants and both products are compounds, so the positive ions traded partners instead.
CA synthesis reaction
This option is wrong — you looked for combining into one product — there are two products, formed by exchanging partners.
DA decomposition reaction
This option is wrong — you looked for one compound breaking apart — there are two reactants, and they exchange partners.
Each side shows two compounds and no uncombined element. The positive ions traded partners: calcium pairs with carbonate, and potassium pairs with chloride. Two dissolved compounds exchanging partners is double replacement.
Check your understanding
Classify the reaction 2Al + 3CuCl₂ → 2AlCl₃ + 3Cu.
AA single replacement reactioncorrect
BA double replacement reaction
This option is wrong — you saw partners changing and jumped to double replacement — double replacement needs TWO COMPOUNDS trading partners, but here an uncombined element, Al, does the replacing.
CA synthesis reaction
This option is wrong — you looked for combining into one product — there are two products, and aluminum takes copper's place.
DA decomposition reaction
This option is wrong — you looked for one compound breaking apart — there are two reactants, one of them an uncombined element.
The reactant side shows an uncombined element, Al, next to a compound. An uncombined element taking the place of an element in a compound is single replacement. Double replacement has two compounds on each side and no uncombined element anywhere.
Check your understanding
Classify the reaction MgCl₂(aq) + 2AgNO₃(aq) → 2AgCl(s) + Mg(NO₃)₂(aq).
AA double replacement reactioncorrect
BA single replacement reaction
This option is wrong — you looked for an uncombined element — every substance in the equation is a compound; the positive ions traded partners.
CA synthesis reaction
This option is wrong — you looked for combining into one product — two new compounds form by exchanging partners.
DA decomposition reaction
This option is wrong — you looked for one compound breaking apart — two dissolved compounds react here.
Two compounds appear on each side, with no uncombined element. The positive ions traded partners: silver pairs with chloride, and magnesium pairs with nitrate. That is the double replacement fingerprint.
Lesson 34 of 51 · RXN-034
Precipitates
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Did You Know?
Wonder this:
Pour two crystal-clear solutions together and, in an instant, the mixture turns cloudy white. A solid is appearing where there was none.
You've read equations in which two dissolved compounds react. Some of those reactions announce themselves exactly like this.
The idea
Mixing two solutions can produce a solid, even though both starting liquids were clear.
A 'precipitate' is a solid that forms and does not dissolve when two solutions are mixed.
A precipitate is a solid that forms and does not dissolve when two solutions are mixed.
In an equation, the precipitate is the product marked (s).
The cloudiness you see is the solid forming throughout the liquid, before it settles to the bottom.
Worked examples
Worked example 1. Two clear solutions are mixed, and a yellow solid appears and settles. What is the solid called?
Step 1
Answer: a precipitate.
Worked example 2. In Na₂CO₃(aq) + CaCl₂(aq) → CaCO₃(s) + 2NaCl(aq), which product is the precipitate?
Step 1
Answer: CaCO₃ — the product marked (s).
You can now state that a precipitate is a solid that forms and does not dissolve when two solutions are mixed.
Check your understanding
What is a precipitate?
AA solid that forms and does not dissolve when two solutions are mixed.correct
BA gas that bubbles out of a mixture of two solutions.
This option is wrong — you swapped states — a precipitate is a SOLID; escaping gas is a different reaction sign.
CA solid that dissolves as soon as it forms.
This option is wrong — you dropped half the definition — a precipitate does NOT dissolve; that is why it stays visible and settles.
DThe clear liquid left behind after a solid settles.
This option is wrong — you named the wrong part of the mixture — the precipitate is the solid itself, not the liquid above it.
A precipitate is a solid that forms and does not dissolve when two solutions are mixed. Because it does not dissolve, it clouds the liquid and then settles.
Check your understanding
In AgNO₃(aq) + KBr(aq) → AgBr(s) + KNO₃(aq), which product is the precipitate?
AAgBrcorrect
BKNO₃
This option is wrong — you picked the product marked (aq) — that compound stays dissolved; the precipitate is the product marked (s).
CAgNO₃
This option is wrong — you picked a reactant — a precipitate is a PRODUCT, the new solid the mixing produces.
DThere is no precipitate in this reaction.
This option is wrong — you missed the (s) — the state symbol marks AgBr as a solid forming from the mixed solutions.
In an equation, the precipitate is the product marked (s). AgBr carries the (s), so AgBr is the precipitate; KNO₃ stays dissolved as (aq).
Check your understanding
A student mixes two clear solutions. The mixture instantly turns cloudy, and a white powder then settles to the bottom. What does the cloudiness show?
AA precipitate forming throughout the liquid.correct
BThe mixture starting to boil.
This option is wrong — you read cloudiness as bubbles — boiling makes gas bubbles, not a settling solid.
CA gas dissolving into the liquid.
This option is wrong — you ran a gas in instead of a solid out — a dissolving gas would not settle as a powder.
DThe water itself freezing into crystals.
This option is wrong — you froze the solvent — nothing here cools the water; the solid is a new compound forming from the mixed solutions.
A precipitate is a solid that forms and does not dissolve when two solutions are mixed. The cloudiness is that solid forming throughout the liquid, before it settles as the powder.
Lesson 35 of 51 · RXN-035
Soluble or insoluble?
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Did You Know?
Some ionic compounds dissolve in water; others, like the precipitates you've just met, do not. A supplied table of rules tells you which is which.
The idea
A compound that dissolves in water is 'soluble'.
A compound that does not dissolve in water is 'insoluble'.
A solubility-rules table summarizes which ionic compounds dissolve — the table is always given to you, never memorized.
To classify a compound, find the rule that mentions its ions, then check the rule's exceptions.
Solubility rules for ionic compounds in water
Rule
1. Compounds of Group 1 metals (Li⁺, Na⁺, K⁺) are soluble.
2. Nitrates (NO₃⁻) are soluble.
3. Chlorides, bromides, and iodides are soluble, except those of silver (Ag⁺) and lead (Pb²⁺).
4. Sulfates (SO₄²⁻) are soluble, except those of barium (Ba²⁺), calcium (Ca²⁺), and lead (Pb²⁺).
5. Carbonates (CO₃²⁻) and hydroxides (OH⁻) are insoluble, except those of Group 1 metals.
The table is always supplied — find the rule that mentions the compound's ions, then check its exceptions.
AgCl: rule 3 says chlorides are soluble EXCEPT those of silver and lead — silver chloride is an exception, so AgCl is insoluble.
Worked examples
Worked example 1. Use the solubility rules to classify potassium carbonate, K₂CO₃, as soluble or insoluble.
Step 1
Find the rule for carbonates: rule 5 says carbonates are insoluble, except those of Group 1 metals.
Step 2
Potassium is a Group 1 metal, so K₂CO₃ is an exception.
Step 3
K₂CO₃ is soluble.
Worked example 2. Use the solubility rules to classify magnesium hydroxide, Mg(OH)₂, as soluble or insoluble.
Step 1
Find the rule for hydroxides: rule 5 says hydroxides are insoluble, except those of Group 1 metals.
Step 2
Magnesium is not a Group 1 metal, so no exception applies.
Step 3
Mg(OH)₂ is insoluble.
You can now classify an ionic compound as soluble, meaning it dissolves in water, or insoluble, meaning it does not, using a supplied solubility-rules table.
Check your understanding
Use the solubility rules shown. Which one of these four compounds is insoluble in water?
Solubility rules for ionic compounds in water
Rule
1. Compounds of Group 1 metals (Li⁺, Na⁺, K⁺) are soluble.
2. Nitrates (NO₃⁻) are soluble.
3. Chlorides, bromides, and iodides are soluble, except those of silver (Ag⁺) and lead (Pb²⁺).
4. Sulfates (SO₄²⁻) are soluble, except those of barium (Ba²⁺), calcium (Ca²⁺), and lead (Pb²⁺).
5. Carbonates (CO₃²⁻) and hydroxides (OH⁻) are insoluble, except those of Group 1 metals.
APbI₂correct
BKNO₃
This option is wrong — you missed two rules that both make this soluble — potassium is a Group 1 metal and nitrates are soluble.
CMgCl₂
This option is wrong — you invented an exception — chlorides are soluble, and magnesium is not on the exception list.
DNa₂SO₄
This option is wrong — you invented an exception — sodium is a Group 1 metal, and sodium is not a sulfate exception.
Check each compound against its rule. Rule 3 makes iodides soluble EXCEPT those of silver and lead. PbI₂ is a lead iodide, so it is the insoluble one.
Check your understanding
Use the solubility rules shown. Which one of these four compounds is soluble in water?
Solubility rules for ionic compounds in water
Rule
1. Compounds of Group 1 metals (Li⁺, Na⁺, K⁺) are soluble.
2. Nitrates (NO₃⁻) are soluble.
3. Chlorides, bromides, and iodides are soluble, except those of silver (Ag⁺) and lead (Pb²⁺).
4. Sulfates (SO₄²⁻) are soluble, except those of barium (Ba²⁺), calcium (Ca²⁺), and lead (Pb²⁺).
5. Carbonates (CO₃²⁻) and hydroxides (OH⁻) are insoluble, except those of Group 1 metals.
ANaOHcorrect
BCu(OH)₂
This option is wrong — you missed the hydroxide rule — hydroxides are insoluble unless the metal is Group 1, and copper is not.
CCaCO₃
This option is wrong — you missed the carbonate rule — carbonates are insoluble unless the metal is Group 1, and calcium is not.
DAgBr
This option is wrong — you missed the exception — bromides are soluble EXCEPT those of silver and lead.
Rule 5 makes hydroxides insoluble, except those of Group 1 metals. Sodium is a Group 1 metal, so NaOH is the exception — it is soluble. Cu(OH)₂, CaCO₃, and AgBr each fall to a rule or an exception that makes them insoluble.
Check your understanding
Use the solubility rules shown to classify calcium sulfate, CaSO₄.
Solubility rules for ionic compounds in water
Rule
1. Compounds of Group 1 metals (Li⁺, Na⁺, K⁺) are soluble.
2. Nitrates (NO₃⁻) are soluble.
3. Chlorides, bromides, and iodides are soluble, except those of silver (Ag⁺) and lead (Pb²⁺).
4. Sulfates (SO₄²⁻) are soluble, except those of barium (Ba²⁺), calcium (Ca²⁺), and lead (Pb²⁺).
5. Carbonates (CO₃²⁻) and hydroxides (OH⁻) are insoluble, except those of Group 1 metals.
AInsoluble — calcium is listed as an exception to the sulfates rule.correct
BSoluble — sulfates are soluble, and calcium is not an exception.
This option is wrong — you stopped reading the rule too early — calcium is on rule 4's exception list, so CaSO₄ is insoluble.
CSoluble — every calcium compound is soluble.
This option is wrong — you invented a calcium rule — the table has no such rule, and calcium is a named sulfate exception.
DInsoluble — all sulfates are insoluble.
This option is wrong — you reached the right verdict with the wrong rule — sulfates are soluble in general; CaSO₄ is insoluble only because calcium is an exception.
Find the rule for sulfates: rule 4 says sulfates are soluble, except those of barium, calcium, and lead. Calcium is on the exception list. So CaSO₄ is insoluble.
Lesson 36 of 51 · RXN-036
Predicting double replacement products
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Did You Know?
You can classify a double replacement reaction. Now write what one produces.
The idea
The positive ions trade partners: each positive ion pairs with the other compound's negative ion.
Write each new compound's formula from its ion charges — never copy subscripts from the old compounds.
Write the positive ion's symbol first in each new formula, as always.
BaCl₂ + Na₂SO₄: barium pairs with sulfate, and sodium pairs with chloride.
Ba²⁺ and SO₄²⁻ give BaSO₄; Na⁺ and Cl⁻ give NaCl.
Trade partners, then write each new formula from the ion charges — never copy old subscripts.
The full balanced equation is BaCl₂ + Na₂SO₄ → BaSO₄ + 2NaCl.
Worked examples
Worked example 1. Solutions of calcium chloride, CaCl₂, and potassium carbonate, K₂CO₃, are mixed and the positive ions trade partners. Calcium is Ca²⁺, potassium is K⁺, carbonate is CO₃²⁻, and chloride is Cl⁻. Predict the products.
Step 1
Trade partners: calcium pairs with carbonate, and potassium pairs with chloride.
Step 2
Ca²⁺ and CO₃²⁻ give CaCO₃.
Step 3
K⁺ and Cl⁻ give KCl.
Step 4
CaCl₂ + K₂CO₃ → CaCO₃ + 2KCl
Worked example 2. Solutions of lead(II) nitrate, Pb(NO₃)₂, and sodium bromide, NaBr, are mixed and the positive ions trade partners. Lead is Pb²⁺, sodium is Na⁺, nitrate is NO₃⁻, and bromide is Br⁻. Predict the products.
Step 1
Trade partners: lead pairs with bromide, and sodium pairs with nitrate.
Step 2
Pb²⁺ and Br⁻ give PbBr₂.
Step 3
Na⁺ and NO₃⁻ give NaNO₃.
Step 4
Pb(NO₃)₂ + 2NaBr → PbBr₂ + 2NaNO₃
You can now predict the products of a double replacement reaction by exchanging the positive ions and writing correct formulas for the new compounds.
Check your understanding
Solutions of magnesium chloride, MgCl₂, and sodium hydroxide, NaOH, are mixed and the positive ions trade partners. Magnesium is Mg²⁺ and hydroxide is OH⁻. Write the formula of the compound formed by the magnesium ions.
Accepted answer: Mg(OH)₂
Magnesium's new partner is hydroxide. Mg²⁺ and OH⁻ give Mg(OH)₂ — the charges set the subscripts. The full equation is MgCl₂ + 2NaOH → Mg(OH)₂ + 2NaCl.
Check your understanding
Solutions of lead(II) nitrate, Pb(NO₃)₂, and potassium sulfate, K₂SO₄, are mixed and the positive ions trade partners. Lead is Pb²⁺, potassium is K⁺, nitrate is NO₃⁻, and sulfate is SO₄²⁻. What are the products?
APbSO₄ and KNO₃correct
BPbSO₄ and K(NO₃)₂
This option is wrong — you copied lead's two-nitrate subscript onto potassium — K⁺ and NO₃⁻ give KNO₃, from the charges alone.
CPb(SO₄)₂ and KNO₃
This option is wrong — you copied the old subscript 2 onto the sulfate — Pb²⁺ and SO₄²⁻ balance one to one, giving PbSO₄.
DPbK and NO₃SO₄
This option is wrong — you paired the two positive ions together — each positive ion pairs with the OTHER compound's negative ion.
Trade partners: lead pairs with sulfate, and potassium pairs with nitrate. Pb²⁺ and SO₄²⁻ give PbSO₄; K⁺ and NO₃⁻ give KNO₃. The full equation is Pb(NO₃)₂ + K₂SO₄ → PbSO₄ + 2KNO₃.
Check your understanding
A student predicts the products of CaBr₂ + Na₂SO₄ and writes one of them as NaBr₂. What went wrong?
AThe student copied the old subscript — sodium is Na⁺ and bromide is Br⁻, so the compound is NaBr.correct
BNothing — NaBr₂ is correct, because calcium bromide contained two bromides.
This option is wrong — you carried the old compound's subscript into the new formula — each new formula comes from its own ion charges, and 1+ with 1− gives NaBr.
CThe symbols are in the wrong order — the formula should be written BrNa.
This option is wrong — you moved the wrong thing — the positive ion is correctly written first; the error is the copied subscript.
DSodium should have paired with calcium instead of with bromide.
This option is wrong — you paired the two positive ions together — each positive ion pairs with the other compound's NEGATIVE ion.
New formulas come from the ion charges, never from the old compounds. Na⁺ and Br⁻ balance one to one, giving NaBr. The full equation is CaBr₂ + Na₂SO₄ → CaSO₄ + 2NaBr.
Lesson 37 of 51 · RXN-037
Predicting the precipitate
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Did You Know?
You can predict a double replacement's products, and you can read a solubility table. Put the two together: which product, if either, is the precipitate?
The idea
Predict the two products by trading partners, as you already do.
Then look up each product in the supplied solubility-rules table.
An insoluble product forms as a solid — it is the precipitate, and it gets the state symbol (s).
Solubility rules for ionic compounds in water
Rule
1. Compounds of Group 1 metals (Li⁺, Na⁺, K⁺) are soluble.
2. Nitrates (NO₃⁻) are soluble.
3. Chlorides, bromides, and iodides are soluble, except those of silver (Ag⁺) and lead (Pb²⁺).
4. Sulfates (SO₄²⁻) are soluble, except those of barium (Ba²⁺), calcium (Ca²⁺), and lead (Pb²⁺).
5. Carbonates (CO₃²⁻) and hydroxides (OH⁻) are insoluble, except those of Group 1 metals.
Predict the products, then look each one up: insoluble means precipitate (s); soluble means dissolved (aq).
A soluble product stays dissolved — it gets (aq).
If both products are soluble, no precipitate forms.
Mixing AgNO₃(aq) and NaCl(aq): the products are AgCl and NaNO₃.
The table makes AgCl insoluble and NaNO₃ soluble, so solid AgCl is the precipitate: AgNO₃(aq) + NaCl(aq) → AgCl(s) + NaNO₃(aq).
Worked examples
Worked example 1. Solutions of lead(II) nitrate, Pb(NO₃)₂, and sodium sulfate, Na₂SO₄, are mixed. Use the solubility rules: which product, if either, is the precipitate?
Step 1
Trade partners: the products are PbSO₄ and NaNO₃.
Step 2
Look up PbSO₄: sulfates are soluble except barium's, calcium's, and lead's — insoluble.
Step 3
Look up NaNO₃: sodium is Group 1 and nitrates are soluble — soluble.
Step 4
PbSO₄ is the precipitate: Pb(NO₃)₂(aq) + Na₂SO₄(aq) → PbSO₄(s) + 2NaNO₃(aq).
Worked example 2. Solutions of potassium chloride, KCl, and sodium nitrate, NaNO₃, are mixed. Use the solubility rules: which product, if either, is the precipitate?
Step 1
Trade partners: the products are KNO₃ and NaCl.
Step 2
Look up KNO₃: potassium is Group 1 and nitrates are soluble — soluble.
Step 3
Look up NaCl: sodium is Group 1 and chlorides are soluble — soluble.
Step 4
Both products are soluble, so no precipitate forms.
You can now predict which product of a double replacement reaction forms as a precipitate, or that no precipitate forms, using a supplied solubility-rules table.
Check your understanding
Solutions of barium chloride, BaCl₂, and potassium sulfate, K₂SO₄, are mixed. Use the solubility rules shown. Which product, if either, is the precipitate?
Solubility rules for ionic compounds in water
Rule
1. Compounds of Group 1 metals (Li⁺, Na⁺, K⁺) are soluble.
2. Nitrates (NO₃⁻) are soluble.
3. Chlorides, bromides, and iodides are soluble, except those of silver (Ag⁺) and lead (Pb²⁺).
4. Sulfates (SO₄²⁻) are soluble, except those of barium (Ba²⁺), calcium (Ca²⁺), and lead (Pb²⁺).
5. Carbonates (CO₃²⁻) and hydroxides (OH⁻) are insoluble, except those of Group 1 metals.
ABaSO₄ is the precipitate.correct
BKCl is the precipitate.
This option is wrong — you marked the soluble product as the solid — potassium is a Group 1 metal and chlorides are soluble, so KCl stays dissolved.
CBoth products are precipitates.
This option is wrong — you skipped the table check on KCl — only the insoluble product forms as a solid.
DNeither — no precipitate forms.
This option is wrong — you missed the sulfate exception — barium is on rule 4's exception list, so BaSO₄ is insoluble.
Trade partners: the products are BaSO₄ and KCl. The table makes BaSO₄ insoluble (barium is a sulfate exception) and KCl soluble. So BaSO₄ is the precipitate: BaCl₂(aq) + K₂SO₄(aq) → BaSO₄(s) + 2KCl(aq).
Check your understanding
Solutions of magnesium sulfate, MgSO₄, and sodium hydroxide, NaOH, are mixed. Use the solubility rules shown. Which product, if either, is the precipitate?
Solubility rules for ionic compounds in water
Rule
1. Compounds of Group 1 metals (Li⁺, Na⁺, K⁺) are soluble.
2. Nitrates (NO₃⁻) are soluble.
3. Chlorides, bromides, and iodides are soluble, except those of silver (Ag⁺) and lead (Pb²⁺).
4. Sulfates (SO₄²⁻) are soluble, except those of barium (Ba²⁺), calcium (Ca²⁺), and lead (Pb²⁺).
5. Carbonates (CO₃²⁻) and hydroxides (OH⁻) are insoluble, except those of Group 1 metals.
AMg(OH)₂ is the precipitate.correct
BNa₂SO₄ is the precipitate.
This option is wrong — you marked the soluble product as the solid — sodium is a Group 1 metal, so Na₂SO₄ stays dissolved.
CBoth products are precipitates.
This option is wrong — you skipped the table check on Na₂SO₄ — only the insoluble product forms as a solid.
DNeither — no precipitate forms.
This option is wrong — you missed the hydroxide rule — hydroxides are insoluble unless the metal is Group 1, and magnesium is not.
Trade partners: the products are Mg(OH)₂ and Na₂SO₄. The table makes Mg(OH)₂ insoluble (rule 5) and Na₂SO₄ soluble (Group 1). So Mg(OH)₂ is the precipitate: MgSO₄(aq) + 2NaOH(aq) → Mg(OH)₂(s) + Na₂SO₄(aq).
Check your understanding
Solutions of sodium bromide, NaBr, and potassium nitrate, KNO₃, are mixed. Use the solubility rules shown. Which product, if either, is the precipitate?
Solubility rules for ionic compounds in water
Rule
1. Compounds of Group 1 metals (Li⁺, Na⁺, K⁺) are soluble.
2. Nitrates (NO₃⁻) are soluble.
3. Chlorides, bromides, and iodides are soluble, except those of silver (Ag⁺) and lead (Pb²⁺).
4. Sulfates (SO₄²⁻) are soluble, except those of barium (Ba²⁺), calcium (Ca²⁺), and lead (Pb²⁺).
5. Carbonates (CO₃²⁻) and hydroxides (OH⁻) are insoluble, except those of Group 1 metals.
ANeither — no precipitate forms.correct
BNaNO₃ is the precipitate.
This option is wrong — you called a soluble product a solid — sodium is Group 1 and nitrates are soluble, so NaNO₃ stays dissolved.
CKBr is the precipitate.
This option is wrong — you called a soluble product a solid — potassium is Group 1 and bromides are soluble, with no exception for potassium.
DBoth products are precipitates.
This option is wrong — you skipped the table entirely — both products land on rules that make them soluble.
Trade partners: the products are NaNO₃ and KBr. The table makes both soluble — Group 1 metals, nitrates, and bromides with no applicable exception. If both products are soluble, no precipitate forms.
Lesson 38 of 51 · RXN-038
Combustion reactions
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Have You Ever Wondered?
Wonder this:
Turn the knob on a gas stove and blue flames leap up. Natural gas is streaming out and vanishing into them — what is it turning into?
You've classified four reaction types. The stove's flame is the fifth and last one.
The idea
Natural gas is mostly methane, CH₄ — a compound of carbon and hydrogen.
A compound made of carbon and hydrogen is a 'hydrocarbon'.
When a hydrocarbon burns, it reacts with oxygen gas to produce carbon dioxide and water.
A reaction in which a hydrocarbon reacts with oxygen gas to produce carbon dioxide and water is a 'combustion' reaction.
The stove's flame is CH₄ + 2O₂ → CO₂ + 2H₂O.
The fingerprint is a hydrocarbon plus O₂ on the left, and CO₂ plus H₂O on the right.
The combustion fingerprint: a hydrocarbon plus O₂ in, CO₂ plus H₂O out.
A metal burning in oxygen, like 2Mg + O₂ → 2MgO, is not combustion in this scheme — two reactants combine into one product, so it is synthesis.
Worked examples
Worked example 1. Classify the reaction 2C₂H₆ + 7O₂ → 4CO₂ + 6H₂O.
Step 1
C₂H₆ is a compound of carbon and hydrogen — a hydrocarbon.
Step 2
It reacts with oxygen gas, and the products are carbon dioxide and water.
Step 3
Combustion — a hydrocarbon reacting with oxygen gas to produce carbon dioxide and water.
Worked example 2. Steel wool burns brightly in oxygen: 4Fe + 3O₂ → 2Fe₂O₃. Classify the reaction.
Step 1
Iron is a metal, not a hydrocarbon, and no carbon dioxide or water forms.
Step 2
Two reactants combine into a single product.
Step 3
Synthesis — burning alone does not make a reaction combustion in this scheme.
You can now classify a reaction as combustion when a compound of carbon and hydrogen, called a hydrocarbon, reacts with oxygen gas to produce carbon dioxide and water.
Check your understanding
Classify the reaction 2C₄H₁₀ + 13O₂ → 8CO₂ + 10H₂O.
AA combustion reactioncorrect
BA synthesis reaction
This option is wrong — you looked for combining into one product — there are two products, and the reactant C₄H₁₀ is a hydrocarbon burning in oxygen.
CA single replacement reaction
This option is wrong — you treated O₂ as a replacing element — nothing is pushed out of a compound; the hydrocarbon and oxygen form carbon dioxide and water.
DA decomposition reaction
This option is wrong — you looked for one compound breaking apart — there are two reactants.
C₄H₁₀ is a compound of carbon and hydrogen — a hydrocarbon. It reacts with oxygen gas, and the products are carbon dioxide and water. That is the combustion fingerprint.
Check your understanding
Classify the reaction 2Ca + O₂ → 2CaO.
AA synthesis reactioncorrect
BA combustion reaction
This option is wrong — you classified by the burning — in this scheme combustion needs a HYDROCARBON producing carbon dioxide and water; a metal burning in oxygen is synthesis.
CA single replacement reaction
This option is wrong — you treated the uncombined elements as replacers — nothing is pushed out of a compound; the two elements combine into one product.
DA decomposition reaction
This option is wrong — you ran the arrow backwards — the single compound is the product, not the starting substance.
Calcium is a metal, not a hydrocarbon, and no carbon dioxide or water forms. Two reactants combine into a single product. That is synthesis — burning alone does not make a reaction combustion.
This option is wrong — you looked for combining into one product — two products form, from a hydrocarbon burning in oxygen.
CA double replacement reaction
This option is wrong — you looked for two compounds trading partners — one reactant is the uncombined element O₂, and the products are carbon dioxide and water.
DA decomposition reaction
This option is wrong — you looked for one compound breaking apart — there are two reactants.
C₈H₁₈ is a compound of carbon and hydrogen — a hydrocarbon. It reacts with oxygen gas, producing carbon dioxide and water. A hydrocarbon plus O₂ giving CO₂ plus H₂O is combustion.
Lesson 39 of 51 · RXN-039
Predicting combustion products
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Did You Know?
You can spot a combustion reaction. Predicting its products is even easier, because every complete combustion makes the same two.
The idea
When a hydrocarbon burns completely in plenty of oxygen, the products are always carbon dioxide and water.
Every carbon atom ends up in a CO₂ molecule.
Every hydrogen atom ends up in an H₂O molecule.
Propane, C₃H₈, burning in plenty of oxygen: the products are CO₂ and H₂O, and the balanced equation is C₃H₈ + 5O₂ → 3CO₂ + 4H₂O.
The prediction holds only for complete combustion, with plenty of oxygen available.
Worked examples
Worked example 1. Hexane, C₆H₁₄, burns completely in plenty of oxygen. Predict the products.
Step 1
Hexane is a hydrocarbon, and the combustion is complete.
Step 2
Every carbon atom ends up in CO₂, and every hydrogen atom ends up in H₂O.
You can now predict the products of the complete combustion of a hydrocarbon as carbon dioxide and water.
Check your understanding
Ethane, C₂H₆, burns completely in plenty of oxygen. What are the products?
ACarbon dioxide and water.correct
BCarbon monoxide and water.
This option is wrong — you swapped in carbon monoxide — in complete combustion with plenty of oxygen, every carbon atom ends up in carbon dioxide, CO₂.
CCarbon and water.
This option is wrong — you left the carbon unburned as soot — with plenty of oxygen, every carbon atom ends up in CO₂.
DCarbon dioxide and hydrogen gas.
This option is wrong — you left the hydrogen uncombined — every hydrogen atom ends up in a water molecule, H₂O.
Ethane is a hydrocarbon burning completely in plenty of oxygen. Every carbon atom ends up in CO₂, and every hydrogen atom ends up in H₂O. The products are carbon dioxide and water: 2C₂H₆ + 7O₂ → 4CO₂ + 6H₂O.
Check your understanding
A hydrocarbon fuel is burning. Which condition must hold for its products to be carbon dioxide and water?
APlenty of oxygen must be available, so the combustion is complete.correct
BThe hydrocarbon must be a gas.
This option is wrong — you added a state requirement — liquid and solid hydrocarbons burn to the same two products when oxygen is plentiful.
CThe hydrocarbon must already contain oxygen.
This option is wrong — you put the oxygen in the wrong place — a hydrocarbon contains only carbon and hydrogen; the oxygen comes from the O₂ it burns in.
DThe temperature must be kept low while it burns.
This option is wrong — you invented a temperature condition — the stated condition is plenty of oxygen for complete combustion.
The prediction holds only for complete combustion. Complete combustion needs plenty of oxygen available. With that condition met, the products are always carbon dioxide and water.
Check your understanding
Heptane, C₇H₁₆, in gasoline burns completely in a car engine running with plenty of oxygen. What are the products?
ACarbon dioxide and water.correct
BCarbon dioxide only.
This option is wrong — you dropped the hydrogen's product — every hydrogen atom ends up in a water molecule.
CWater only.
This option is wrong — you dropped the carbon's product — every carbon atom ends up in a carbon dioxide molecule.
DCarbon dioxide, water, and carbon monoxide.
This option is wrong — you added carbon monoxide — with plenty of oxygen, every carbon atom ends up in carbon dioxide, so only two products form.
Heptane is a hydrocarbon burning completely in plenty of oxygen. Every carbon atom ends up in CO₂, and every hydrogen atom ends up in H₂O. The products are carbon dioxide and water — nothing else.
Lesson 40 of 51 · RXN-040
Classifying the five reaction types
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One at a time, five reaction types are behind you: synthesis, decomposition, single replacement, double replacement, and combustion. A real equation arrives without a label. This lesson builds the routine that sorts any equation into its type.
The idea
Read the whole equation before you classify: count the reactants, count the products, and note which formulas are elements and which are compounds.
Check for combustion first: a hydrocarbon reacting with O₂ to form CO₂ and H₂O is a combustion.
If two or more reactants combine into a single product, the reaction is a synthesis — 2Na + Cl₂ → 2NaCl is a synthesis.
A metal burning in oxygen fits the synthesis pattern, not the combustion pattern, because no hydrocarbon is involved.
If a single compound breaks apart into two or more simpler substances, the reaction is a decomposition — CaCO₃ → CaO + CO₂ is a decomposition.
If an uncombined element takes the place of an element in a compound, the reaction is a single replacement — Zn + 2HCl → ZnCl₂ + H₂ is a single replacement.
If two compounds dissolved in water exchange partners to form two new compounds, the reaction is a double replacement — KCl(aq) + AgNO₃(aq) → AgCl(s) + KNO₃(aq) is a double replacement.
Run the checks in this order until one pattern fits: every equation you are asked to classify in this lesson fits exactly one of the five types — some real reactions fit none of them, and you will meet a few later in the unit.
The five patterns sit side by side in the table.
The five reaction types
Type
Pattern
Example
Combustion
hydrocarbon + O₂ → CO₂ + H₂O
CH₄ + 2O₂ → CO₂ + 2H₂O
Synthesis
two or more reactants → one product
2Na + Cl₂ → 2NaCl
Decomposition
one compound → two or more substances
CaCO₃ → CaO + CO₂
Single replacement
element + compound → new element + new compound
Zn + 2HCl → ZnCl₂ + H₂
Double replacement
two dissolved compounds exchange partners
KCl(aq) + AgNO₃(aq) → AgCl(s) + KNO₃(aq)
Check combustion first, then match the reactant-product pattern.
Worked examples
Worked example 1. Classify the reaction 2C₂H₆ + 7O₂ → 4CO₂ + 6H₂O.
Step 1
Check combustion first: C₂H₆ is a hydrocarbon, it reacts with O₂, and the products are CO₂ and H₂O.
Step 2
The reaction is a combustion.
Worked example 2. Classify the reaction Fe + CuSO₄ → FeSO₄ + Cu.
Step 1
No hydrocarbon burns, more than one product forms, and no compound breaks apart alone.
Step 2
Uncombined Fe takes copper's place in CuSO₄, leaving uncombined Cu.
Step 3
The reaction is a single replacement.
Worked example 3. Classify the reaction Pb(NO₃)₂(aq) + 2KI(aq) → PbI₂(s) + 2KNO₃(aq).
Step 1
Every formula on both sides is a compound — no uncombined element appears anywhere.
Step 2
The two reactant compounds, dissolved in water, exchange partners: lead pairs with iodide, potassium pairs with nitrate.
Step 3
The reaction is a double replacement.
You can now classify a reaction as synthesis, decomposition, single replacement, double replacement, or combustion from its chemical equation.
Check your understanding
Classify the reaction 2H₂O₂ → 2H₂O + O₂.
ADecompositioncorrect
BSynthesis
This option is wrong — you read the pattern backwards — one compound breaking into two substances is a decomposition; a synthesis combines reactants into one product.
CSingle replacement
This option is wrong — you saw the uncombined O₂ and assumed a replacement — nothing traded places, because only one reactant exists.
DCombustion
This option is wrong — you matched on the oxygen — a combustion needs a hydrocarbon reacting WITH O₂, and here O₂ is a product.
Count the reactants: only one, the compound H₂O₂. One compound breaking apart into two or more simpler substances is a decomposition. 2H₂O₂ → 2H₂O + O₂ is a decomposition.
Check your understanding
Classify the reaction C₃H₈ + 5O₂ → 3CO₂ + 4H₂O.
ACombustioncorrect
BSynthesis
This option is wrong — you counted two reactants heading toward products — but two products form here, and the hydrocarbon-plus-O₂-gives-CO₂-and-H₂O pattern is checked first.
CDouble replacement
This option is wrong — you treated the four formulas as partner-swapping compounds — C₃H₈ burns in O₂, an uncombined element, and the products are the fixed combustion pair CO₂ and H₂O.
DDecomposition
This option is wrong — you focused on the hydrocarbon breaking up — a decomposition has ONE reactant, and this reaction has two.
Check combustion first: C₃H₈ is a hydrocarbon and it reacts with O₂. The products are CO₂ and H₂O — the full combustion pattern. C₃H₈ + 5O₂ → 3CO₂ + 4H₂O is a combustion.
Check your understanding
Classify the reaction Mg + 2HCl → MgCl₂ + H₂.
ASingle replacementcorrect
BDouble replacement
This option is wrong — you counted two swaps — only the uncombined element Mg traded places with hydrogen; a double replacement needs two COMPOUNDS exchanging partners.
CSynthesis
This option is wrong — you saw magnesium combining with chlorine — but two products form, and a synthesis makes exactly one.
DDecomposition
This option is wrong — you focused on HCl splitting up — a decomposition has one reactant, and this reaction has two.
Uncombined Mg takes hydrogen's place in the compound HCl. The products are a new compound, MgCl₂, and a new uncombined element, H₂. Element replaces element in a compound: a single replacement.
Lesson 41 of 51 · RXN-041
Explaining reaction outcomes
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You can already predict WHAT forms when sodium meets chlorine. This lesson builds the explanation of WHY that product forms, using two things you can read from the periodic table: each element's group and its valence electrons.
The idea
To explain a reaction's outcome, start from each reactant's position in the periodic table.
The group number gives the valence electrons: sodium sits in Group 1, so a sodium atom has one valence electron.
Chlorine sits in Group 17, so a chlorine atom has seven valence electrons and gains one more to complete its outer shell at eight.
When the two elements react, each sodium atom gives its one valence electron to a chlorine atom.
One sodium atom supplies exactly the one electron one chlorine atom gains, so the atoms pair one-to-one, and the product is NaCl.
Each sodium atom gives its one valence electron to a chlorine atom, so the atoms pair one-to-one.
That is the whole explanation template: group position, then valence electrons, then who gives and who gains, then how many of each atom pair up, then the product's formula.
Worked examples
Worked example 1. Magnesium (Group 2) reacts with oxygen (Group 16) to form MgO. Explain why the product contains one magnesium for every oxygen.
Step 1
Magnesium is in Group 2, so each magnesium atom gives two valence electrons.
Step 2
Oxygen is in Group 16, so each oxygen atom gains two electrons to complete its outer shell.
Step 3
One magnesium atom supplies exactly the two electrons one oxygen atom gains.
Step 4
The atoms pair one-to-one, so the product is MgO.
Worked example 2. Potassium (Group 1) reacts with sulfur (Group 16) to form K₂S. Explain why two potassium atoms are needed for each sulfur atom.
Step 1
Potassium is in Group 1, so each potassium atom gives one valence electron.
Step 2
Sulfur is in Group 16, so each sulfur atom gains two electrons to complete its outer shell.
Step 3
One potassium atom supplies only one electron, so two potassium atoms are needed to supply the two electrons one sulfur atom gains.
Step 4
Two potassium atoms pair with each sulfur atom, so the product is K₂S.
You can now explain the outcome of a simple chemical reaction using the reactants' valence electrons and positions in the periodic table.
Check your understanding
Calcium (Group 2) reacts with fluorine (Group 17) to form CaF₂. Which explanation is correct?
AEach calcium atom gives two electrons, each fluorine atom gains one, so two fluorine atoms pair with each calcium atom.correct
BEach calcium atom gains two electrons, each fluorine atom gives one, so two fluorine atoms pair with each calcium atom.
This option is wrong — you ran the transfer backwards — the metal gives its valence electrons and the nonmetal gains them.
CEach calcium atom gives two electrons, each fluorine atom gains two, so the atoms pair one-to-one.
This option is wrong — you gave fluorine the Group 16 gain — a Group 17 atom gains ONE electron, so two fluorine atoms are needed per calcium.
DEach calcium atom gives one electron, each fluorine atom gains one, so the atoms pair one-to-one.
This option is wrong — you gave calcium the Group 1 count — a Group 2 metal has two valence electrons to give.
Group 2: each calcium atom gives two valence electrons. Group 17: each fluorine atom gains one electron to complete its outer shell. Two fluorine atoms take the two electrons one calcium atom gives, so the product is CaF₂.
Check your understanding
Lithium (Group 1) reacts with oxygen (Group 16) to form Li₂O. Why are two lithium atoms needed for each oxygen atom?
AEach lithium atom gives only one electron, and each oxygen atom gains two, so two lithium atoms supply what one oxygen atom gains.correct
BEach lithium atom gives two electrons, and each oxygen atom gains four, so two lithium atoms supply what one oxygen atom gains.
This option is wrong — you doubled both counts — Group 1 gives one electron and Group 16 gains two; the 2 in Li₂O comes from needing two one-electron givers.
CLithium atoms are smaller than oxygen atoms, so two of them fit around each oxygen atom.
This option is wrong — you explained the ratio by size — the ratio comes from electron bookkeeping: two given electrons must equal the two gained.
DEach oxygen atom gives two electrons, and each lithium atom gains one, so two lithium atoms are needed per oxygen.
This option is wrong — you ran the transfer backwards — the metal lithium gives electrons and the nonmetal oxygen gains them.
Group 1: each lithium atom gives one valence electron. Group 16: each oxygen atom gains two electrons to complete its outer shell. Two lithium atoms supply the two electrons one oxygen atom gains, so the product is Li₂O.
Check your understanding
Aluminum (Group 13) reacts with chlorine (Group 17) to form AlCl₃. Which explanation is correct?
AEach aluminum atom gives three electrons, each chlorine atom gains one, so three chlorine atoms pair with each aluminum atom.correct
BEach aluminum atom gives one electron, each chlorine atom gains three, so three chlorine atoms pair with each aluminum atom.
This option is wrong — you swapped the two counts — Group 13 gives three electrons and Group 17 gains one.
CEach aluminum atom gains three electrons from three chlorine atoms, completing aluminum's outer shell.
This option is wrong — you ran the transfer backwards — the metal aluminum gives its valence electrons and the nonmetal chlorine gains them.
DEach aluminum atom gives three electrons, each chlorine atom gains three, so the atoms pair one-to-one.
This option is wrong — you matched chlorine's gain to aluminum's give — a Group 17 atom gains only ONE electron, so three chlorine atoms are needed.
Group 13: each aluminum atom gives three valence electrons. Group 17: each chlorine atom gains one electron to complete its outer shell. Three chlorine atoms take the three electrons one aluminum atom gives, so the product is AlCl₃.
Summary video — The five reaction types and predicting products
Every chemistry stockroom keeps two families of hazards on separate shelves: acids and bases. The bottle labels spell the names out — hydrochloric acid, sodium hydroxide — but an equation shows you only formulas. You need a way to spot an acid or a base from the formula alone.
Two formula patterns do the job for the acids and bases you will meet in this course.
The idea
Common laboratory acids have formulas that begin with H.
HCl (hydrochloric acid), HNO₃ (nitric acid), and H₂SO₄ (sulfuric acid) are the acids you will meet most often — each formula begins with H, and each compound is an 'acid'.
Common laboratory bases are metal compounds whose formulas end in OH.
NaOH (sodium hydroxide) and Ca(OH)₂ (calcium hydroxide) are the bases you will meet most often — each is a metal joined to OH, and each compound is a 'base'.
The two patterns are recognition shortcuts for common laboratory acids and bases only — they are not definitions.
Spotting acids and bases from formulas
Pattern
Examples
Not covered
Acid: formula begins with H
HCl, HNO₃, H₂SO₄
H₂O is not an acid
Base: metal compound ending in OH
NaOH, Ca(OH)₂
CH₃OH has no metal, so it is not a base
Two recognition shortcuts, valid for common laboratory acids and bases only.
H₂O begins with H, but water is not one of the laboratory acids — keep water off both lists.
CH₃OH ends in OH, but no metal is attached — a carbon compound carrying OH is not a base.
Containing hydrogen somewhere in the formula is not enough either — CH₄ is neither an acid nor a base.
Worked examples
Worked example 1. Is KOH an acid, a base, or neither?
Step 1
KOH ends in OH, and K is a metal — potassium.
Step 2
KOH is a base (potassium hydroxide).
Worked example 2. Is HBr an acid, a base, or neither?
Step 1
HBr begins with H, and it is one of the common laboratory acids — hydrobromic acid.
Step 2
HBr is an acid.
You can now identify a compound as a common acid or base from its formula, where common laboratory acids have formulas beginning with H and common bases are metal compounds ending in OH.
Check your understanding
H₃PO₄ is used in some soft drinks. Classify H₃PO₄.
AAn acidcorrect
BA base
This option is wrong — you matched the wrong pattern — a base is a metal compound ending in OH, and H₃PO₄ begins with H, the acid pattern.
CBoth an acid and a base
This option is wrong — you let one compound satisfy both patterns — H₃PO₄ begins with H but is not a metal compound ending in OH.
DNeither an acid nor a base
This option is wrong — you missed the leading H — a formula beginning with H marks a common laboratory acid, and H₃PO₄ (phosphoric acid) is one.
Check the acid pattern first: H₃PO₄ begins with H. It is one of the common laboratory acids — phosphoric acid. H₃PO₄ is an acid.
Check your understanding
LiOH is used in spacecraft air scrubbers. Classify LiOH.
AA basecorrect
BAn acid
This option is wrong — you matched the wrong pattern — an acid's formula begins with H, and LiOH begins with the metal Li.
CNeither an acid nor a base
This option is wrong — you missed the base pattern — LiOH is a metal (lithium) joined to OH, which marks a common laboratory base.
DBoth an acid and a base
This option is wrong — you counted the H inside OH as a leading H — the acid pattern needs the formula to BEGIN with H, and LiOH begins with Li.
Check the base pattern: LiOH ends in OH. Li is a metal — lithium — so the OH sits on a metal. LiOH is a base (lithium hydroxide).
Check your understanding
C₂H₅OH is the alcohol in hand sanitizer. Classify C₂H₅OH.
ANeither an acid nor a basecorrect
BA base
This option is wrong — you matched on the OH ending alone — the OH must sit on a METAL, and C₂H₅OH is a carbon compound.
CAn acid
This option is wrong — you matched on the H atoms in the formula — the acid pattern needs the formula to BEGIN with H, and C₂H₅OH begins with C.
DBoth an acid and a base
This option is wrong — you let the H atoms and the OH ending each count — neither pattern fits: no leading H, and no metal carrying the OH.
Acid check: the formula begins with C, not H. Base check: it ends in OH, but no metal is attached — C₂H₅OH is a carbon compound. C₂H₅OH is neither an acid nor a base.
Lesson 43 of 51 · RXN-043
Recognizing acid-base reactions
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Did You Know?
Acids and bases are now recognizable from their formulas. When one of each reacts, the equation follows one fixed pattern — and that pattern is how you recognize an acid-base reaction on sight.
The idea
An 'acid-base reaction' is an acid and a base reacting to form water and an ionic compound.
Check the reactant side first: one reactant begins with H, and the other is a metal compound ending in OH.
Check the product side next: one product is H₂O, and the other is an ionic compound.
HCl + NaOH → NaCl + H₂O shows the full pattern: acid HCl, base NaOH, ionic compound NaCl, and water.
If either check fails — no acid, no base, or no water among the products — the reaction is not an acid-base reaction.
All the bases in this course are metal compounds ending in OH — that is why the products here are always water plus an ionic compound. (Chemistry has other bases too; they wait for a later course.)
Worked examples
Worked example 1. Is HNO₃ + KOH → KNO₃ + H₂O an acid-base reaction?
Step 1
Reactant check: HNO₃ begins with H — an acid; KOH is a metal compound ending in OH — a base.
Step 2
Product check: H₂O appears, alongside the ionic compound KNO₃.
Step 3
Both checks pass — the reaction is an acid-base reaction.
Worked example 2. Is Zn + 2HCl → ZnCl₂ + H₂ an acid-base reaction?
Step 1
Reactant check: HCl is an acid, but Zn is an uncombined metal, not a base.
Step 2
Product check: the products include H₂, not H₂O.
Step 3
Both checks fail — an acid alone does not make an acid-base reaction.
You can now identify an acid-base reaction from its equation as an acid and a base reacting to form water and an ionic compound.
Check your understanding
Which equation shows an acid-base reaction?
AH₂SO₄ + 2KOH → K₂SO₄ + 2H₂Ocorrect
BNa₂CO₃ + CaCl₂ → CaCO₃ + 2NaCl
This option is wrong — you matched two compounds swapping partners — no reactant is an acid or a base here, and no water forms.
CMg + 2HCl → MgCl₂ + H₂
This option is wrong — you treated any reaction containing an acid as acid-base — the second reactant must be a base, and Mg is an uncombined metal.
D2H₂ + O₂ → 2H₂O
This option is wrong — you matched on water alone — the water must come from an acid reacting with a base, and neither reactant here is either one.
Reactant check: H₂SO₄ begins with H (acid) and KOH is a metal-OH compound (base). Product check: water forms, alongside the ionic compound K₂SO₄. Acid + base → ionic compound + water — the full acid-base pattern.
Check your understanding
Which equation shows an acid-base reaction?
AHCl + LiOH → LiCl + H₂Ocorrect
BCH₄ + 2O₂ → CO₂ + 2H₂O
This option is wrong — you matched on the water among the products — the reactants must be an acid and a base, and here a hydrocarbon burns in oxygen.
CAgNO₃ + KCl → AgCl + KNO₃
This option is wrong — you matched partner-swapping compounds — no reactant begins with H and none is a metal-OH compound, and no water forms.
D2Na + Cl₂ → 2NaCl
This option is wrong — you matched on the ionic compound forming — an acid-base reaction also needs water among the products, made from an acid and a base.
Reactant check: HCl is an acid and LiOH is a base. Product check: water forms, alongside the ionic compound LiCl. HCl + LiOH → LiCl + H₂O fits the acid-base pattern.
Check your understanding
An acid reacting with a metal-OH base always forms which pair of products?
AWater and an ionic compoundcorrect
BWater and a gas
This option is wrong — you swapped in a gas — the partner product is an ionic compound, formed from the acid's and base's leftover ions.
CA solid precipitate and an ionic compound
This option is wrong — you described a precipitation reaction — an acid-base reaction's marker product is water.
DTwo uncombined elements
This option is wrong — you described a decomposition into elements — an acid and a base regroup into water and an ionic compound.
An acid-base reaction is an acid and a base reacting to form water and an ionic compound. The water is the marker product; the ionic compound forms from the remaining ions. Both products must appear for the pattern to fit.
Lesson 44 of 51 · RXN-044
Recognizing precipitation reactions
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Did You Know?
A precipitate — a solid that forms and does not dissolve when two solutions are mixed — already has its name. This lesson gives the reaction its category: how to recognize a precipitation reaction from its equation.
The idea
A 'precipitation reaction' is two dissolved compounds reacting to form a solid product.
The state symbols carry the whole test: both reactants are marked (aq), and a product is marked (s).
AgNO₃(aq) + NaCl(aq) → AgCl(s) + NaNO₃(aq) is a precipitation reaction: two dissolved compounds in, solid AgCl out.
The product marked (s) is the precipitate.
If no product carries (s), no precipitate formed, and the reaction is not a precipitation reaction.
Worked examples
Worked example 1. Is Na₂CO₃(aq) + CaCl₂(aq) → CaCO₃(s) + 2NaCl(aq) a precipitation reaction? If so, name the precipitate.
Step 1
Reactant check: both reactants are marked (aq) — two dissolved compounds.
Step 2
Product check: CaCO₃ is marked (s) — a solid formed.
Step 3
It is a precipitation reaction, and CaCO₃ is the precipitate.
Worked example 2. Is HCl(aq) + KOH(aq) → KCl(aq) + H₂O(l) a precipitation reaction?
Step 1
Reactant check: both reactants are dissolved — the check passes.
Step 2
Product check: no product is marked (s) — everything stays dissolved or liquid.
Step 3
No solid forms, so it is not a precipitation reaction.
You can now identify a precipitation reaction from its equation as two dissolved compounds forming a solid product.
This option is wrong — you counted the fizzing product — CO₂ is a gas, marked (g); a precipitation reaction forms a solid, marked (s).
Reactant check: Ba(NO₃)₂ and K₂SO₄ are both marked (aq). Product check: BaSO₄ is marked (s) — a solid formed from the mixed solutions. Two dissolved compounds forming a solid product: a precipitation reaction.
Check your understanding
In the reaction Pb(NO₃)₂(aq) + 2KI(aq) → PbI₂(s) + 2KNO₃(aq), which substance is the precipitate?
APbI₂correct
BKNO₃
This option is wrong — you picked the other product — the precipitate is the product marked (s), and KNO₃ stays dissolved as (aq).
CPb(NO₃)₂
This option is wrong — you picked a reactant — the precipitate is a PRODUCT, the new solid the mixing creates.
DKI
This option is wrong — you picked a reactant — both reactants are dissolved compounds; the precipitate is the (s) product they form.
The precipitate is the product marked (s). PbI₂ carries the (s); KNO₃ stays dissolved as (aq). PbI₂ is the precipitate.
This option is wrong — you may have expected the base to guarantee a solid — every product here stays dissolved or liquid, with no (s) anywhere.
CFe(s) + 2HCl(aq) → FeCl₂(aq) + H₂(g)
This option is wrong — you counted the solid REACTANT — a precipitation reaction needs two dissolved reactants and a solid product, and here the only solid is consumed.
DC₂H₅OH(l) + 3O₂(g) → 2CO₂(g) + 3H₂O(g)
This option is wrong — you picked a burning reaction — its products are gases, and its reactants are not two dissolved compounds.
Reactant check: CuSO₄ and KOH are both marked (aq). Product check: Cu(OH)₂ carries the (s) — the solid the mixing creates. Two dissolved compounds forming a solid product: a precipitation reaction.
Lesson 45 of 51 · RXN-045
Redox means electron transfer
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Wonder this:
Hang a coil of copper wire in a silver nitrate solution. Glittering silver crystals grow on the wire, and the solution slowly turns blue. Nothing visible passed between the two metals — but something did.
You've already seen that atoms form ions by losing or gaining electrons. Some whole reactions run on exactly that hand-off.
The idea
In some reactions, electrons move from one substance to another.
A reaction in which electrons transfer from one substance to another is an 'oxidation-reduction reaction', also called a 'redox reaction'.
When zinc reacts with copper(II) ions, each zinc atom gives two electrons to a copper ion.
The zinc atoms become Zn²⁺ ions, and the copper ions become copper atoms — the electron transfer IS the reaction.
Each zinc atom gives two electrons to a copper ion — the transfer is the reaction.
You cannot see the electrons move; what you see is the new copper metal appearing and the blue color fading.
Worked examples
Worked example 1. What moves from one substance to another in every oxidation-reduction reaction?
Step 1
Answer: electrons.
Worked example 2. Magnesium burning in oxygen is a redox reaction in which the magnesium atoms give the electrons. Which substance receives them?
Step 1
Answer: oxygen.
You can now state that in an oxidation-reduction reaction, also called a redox reaction, electrons transfer from one substance to another.
Check your understanding
In every oxidation-reduction reaction, what transfers from one substance to another?
AElectronscorrect
BProtons
This option is wrong — you moved particles out of the nucleus — the nucleus stays intact in a chemical reaction; the particles that move between substances are electrons.
CNeutrons
This option is wrong — you moved particles out of the nucleus — neutrons never leave the nucleus in a chemical reaction; electrons are what transfer.
DWhole atoms
This option is wrong — you confused regrouping with the defining transfer — atoms regroup in EVERY reaction, but a redox reaction is defined by electrons moving between substances.
A redox reaction is a reaction in which electrons transfer from one substance to another. The nuclei — protons and neutrons — stay put. Electrons are the transferred particles.
Check your understanding
Iron reacting with chlorine is an oxidation-reduction reaction. What happens between the iron atoms and the chlorine atoms?
AThe iron atoms give electrons to the chlorine atoms.correct
BThe chlorine atoms give electrons to the iron atoms.
This option is wrong — you ran the transfer backwards — the metal gives its electrons and the nonmetal gains them.
CThe iron atoms give protons to the chlorine atoms.
This option is wrong — you transferred the wrong particle — protons stay locked in the nucleus; electrons are what move in a redox reaction.
DNo particles pass between them; the atoms only change positions.
This option is wrong — you removed the defining transfer — a redox reaction happens precisely because electrons move from one substance to the other.
A redox reaction runs on electron transfer. The metal iron gives electrons; the nonmetal chlorine gains them. Iron atoms give electrons to chlorine atoms.
Check your understanding
What name is given to a reaction in which electrons transfer from one substance to another?
AAn oxidation-reduction (redox) reactioncorrect
BA precipitation reaction
This option is wrong — you named the solid-forming category — a precipitation reaction is defined by a solid forming from two solutions, not by electron transfer.
CA synthesis reaction
This option is wrong — you named a reaction TYPE — synthesis describes the reactant-product pattern, while the electron-transfer definition belongs to redox.
DA decomposition reaction
This option is wrong — you named a reaction TYPE — decomposition describes one compound breaking apart, while the electron-transfer definition belongs to redox.
The definition is the name's whole content: electrons transferring between substances. That reaction is an oxidation-reduction reaction. 'Redox' is its shorter name.
Lesson 46 of 51 · RXN-046
Recognizing redox reactions
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Electron transfer is invisible on paper — an equation never shows the electrons moving. This lesson gives the visible shortcut: how to spot an oxidation-reduction reaction from the formulas alone.
The idea
Look for an element changing between its uncombined form and being part of a compound.
If an uncombined element becomes part of a compound, the reaction is a redox reaction.
If an element inside a compound becomes uncombined, the reaction is also a redox reaction.
Every combustion is a redox reaction too.
Zn + CuSO₄ → ZnSO₄ + Cu shows both directions at once: uncombined Zn becomes part of ZnSO₄, and the copper in CuSO₄ becomes uncombined Cu.
The shortcut works because an element changing between uncombined and combined form always involves electron transfer.
Redox reactions with no uncombined element exist, but they are beyond this course — every redox reaction you classify here shows an uncombined element on one side, or is a combustion.
Worked examples
Worked example 1. Is 4Al + 3O₂ → 2Al₂O₃ a redox reaction?
Step 1
Al and O₂ are uncombined elements on the reactant side.
Step 2
Both end up inside the compound Al₂O₃.
Step 3
Uncombined elements became part of a compound — it is a redox reaction.
Worked example 2. Is BaCl₂(aq) + Na₂SO₄(aq) → BaSO₄(s) + 2NaCl(aq) a redox reaction?
Step 1
Every formula on both sides is a compound — no uncombined element appears anywhere.
Step 2
The reaction is not a combustion either.
Step 3
Neither check fits — it is not classified as a redox reaction.
You can now identify a reaction as oxidation-reduction when an uncombined element becomes part of a compound or an element in a compound becomes uncombined, or when the reaction is a combustion.
Check your understanding
Which equation shows an oxidation-reduction reaction?
A2K + Br₂ → 2KBrcorrect
BKOH + HNO₃ → KNO₃ + H₂O
This option is wrong — you picked an acid-base reaction — every substance on both sides is a compound, so no element changes between uncombined and combined form.
CPb(NO₃)₂(aq) + K₂SO₄(aq) → PbSO₄(s) + 2KNO₃(aq)
This option is wrong — you picked a precipitation reaction — the compounds only swap partners, and no uncombined element appears on either side.
DCaCO₃ → CaO + CO₂
This option is wrong — you treated a decomposition as automatically redox — this one breaks a compound into two compounds, with no uncombined element anywhere.
Scan for uncombined elements: K and Br₂ stand alone on the reactant side. Both become part of the compound KBr. An uncombined element becoming part of a compound marks a redox reaction.
Check your understanding
Mg + 2HCl → MgCl₂ + H₂ is a redox reaction. Which feature of the equation shows it?
AUncombined Mg becomes part of a compound, and the hydrogen in HCl becomes uncombined H₂.correct
BA gas forms during the reaction.
This option is wrong — you used a reaction sign — gas formation is evidence that some reaction happened, while the redox marker is an element changing between uncombined and combined form.
COne of the reactants is an acid.
This option is wrong — you matched the acid to a different category — an acid in the equation points toward acid-base checks, and the redox marker is the element changing form.
DThe chlorine atoms change partners, moving from the hydrogen over to the magnesium.
This option is wrong — you tracked the swap instead of the form change — partners change in many non-redox reactions; the marker is an element moving between uncombined and combined form.
Scan for elements changing form. Mg enters uncombined and ends up inside MgCl₂; hydrogen enters inside HCl and leaves as uncombined H₂. Both directions of the element test appear — a redox reaction.
Check your understanding
Which equation shows an oxidation-reduction reaction?
A2H₂O → 2H₂ + O₂correct
BH₂SO₄ + 2NaOH → Na₂SO₄ + 2H₂O
This option is wrong — you picked an acid-base reaction — every substance stays a compound, so no element changes between uncombined and combined form.
CAgNO₃ + KI → AgI + KNO₃
This option is wrong — you picked partner-swapping compounds — no uncombined element appears on either side.
DMgCO₃ → MgO + CO₂
This option is wrong — you treated a decomposition as automatically redox — this one breaks a compound into two compounds, with no uncombined element anywhere.
Scan the water-splitting equation: H₂O is a compound, and H₂ and O₂ are uncombined elements. Elements inside a compound became uncombined. That form change marks a redox reaction.
Lesson 47 of 51 · RXN-047
Differentiating the three categories
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Acid-base, precipitation, and oxidation-reduction are each recognizable on their own. A test question hands you one equation and all three names at once. This lesson builds the sorting routine — and settles how these categories fit alongside the five reaction types.
The idea
Run three checks on the equation, one category at a time.
Acid-base check: an acid and a base react, and the products are water and an ionic compound.
Precipitation check: two dissolved compounds react, and a product is marked (s).
Redox check: an element changes between uncombined and combined form, or the reaction is a combustion.
Exactly one check fits each equation you will meet here; if none fits, the reaction belongs to none of the three categories.
The three reaction categories
Category
What to check for
Acid-base
acid + base react; products are water + an ionic compound
Precipitation
two (aq) reactants; a product marked (s)
Oxidation-reduction
an element changes between uncombined and combined form, or the reaction is a combustion
Run the three checks one at a time; exactly one fits, or none does.
The three categories are a different lens from the five types, so one reaction can carry a label from each lens.
2Mg + O₂ → 2MgO is a synthesis by type AND a redox reaction by category: two reactants form one product, and uncombined Mg and O₂ become part of a compound.
CaCO₃ → CaO + CO₂ is a decomposition by type and none of the three by category: no acid, no base, no precipitate, and no element changing form.
Worked examples
Worked example 1. Classify HBr + KOH → KBr + H₂O as acid-base, precipitation, or oxidation-reduction.
Step 1
Acid-base check: HBr begins with H (acid), KOH is a metal-OH compound (base), and the products are water plus the ionic compound KBr.
Step 2
The acid-base check fits — an acid-base reaction.
Worked example 2. Classify Pb(NO₃)₂(aq) + Na₂SO₄(aq) → PbSO₄(s) + 2NaNO₃(aq).
Step 1
Acid-base check: no reactant is an acid or a base, and no water forms.
Step 2
Precipitation check: two (aq) reactants, and PbSO₄ carries the (s).
Step 3
The precipitation check fits — a precipitation reaction.
Worked example 3. Classify Cu + 2AgNO₃ → Cu(NO₃)₂ + 2Ag.
Step 1
Acid-base check: no acid, no base, no water formed.
Step 2
Precipitation check: one reactant is an uncombined metal, not a dissolved compound.
Step 3
Redox check: uncombined Cu becomes part of a compound, and the silver in AgNO₃ becomes uncombined Ag.
Step 4
The redox check fits — an oxidation-reduction reaction.
You can now differentiate among acid-base, precipitation, and oxidation-reduction reactions from their chemical equations.
Check your understanding
Classify the reaction HF(aq) + NaOH(aq) → NaF(aq) + H₂O(l).
AAn acid-base reactioncorrect
BA precipitation reaction
This option is wrong — you skipped the product states — every product stays dissolved or liquid, so no precipitate formed; the acid, base, and water are the fitting signature.
CAn oxidation-reduction reaction
This option is wrong — you saw new pairings and assumed electron transfer — no element changes between uncombined and combined form, and nothing burns.
DNone of the three categories
This option is wrong — you may have run the checks too strictly — HF is an acid, NaOH is a base, and the products are water plus the ionic compound NaF: the acid-base check fits.
Acid-base check: HF begins with H, NaOH is a metal-OH compound. The products are water and the ionic compound NaF. The acid-base check fits.
Check your understanding
Classify the reaction 2AgNO₃(aq) + MgCl₂(aq) → 2AgCl(s) + Mg(NO₃)₂(aq).
AA precipitation reactioncorrect
BAn acid-base reaction
This option is wrong — you skipped the reactant patterns — neither reactant is an acid or a base, and no water forms among the products.
CAn oxidation-reduction reaction
This option is wrong — you saw the solid appear and assumed electron transfer — the silver stays combined throughout, moving from AgNO₃ into AgCl; no element changes form.
DNone of the three categories
This option is wrong — you may have missed the state symbols — two (aq) reactants formed the (s) product AgCl, which is exactly the precipitation signature.
Precipitation check: both reactants are marked (aq). AgCl carries the (s) — a solid formed from the mixed solutions. The precipitation check fits.
Check your understanding
Classify the reaction Ni + Cl₂ → NiCl₂.
AAn oxidation-reduction reactioncorrect
BAn acid-base reaction
This option is wrong — you skipped the reactant patterns — no reactant is an acid or a base, and no water forms.
CA precipitation reaction
This option is wrong — you matched the solid product — a precipitation reaction needs two DISSOLVED compounds reacting, and these reactants are uncombined elements.
DNone of the three categories
This option is wrong — you may have looked only for burning — the redox check also fires when an uncombined element becomes part of a compound, and both Ni and Cl₂ do exactly that.
Redox check: Ni and Cl₂ are uncombined elements. Both become part of the compound NiCl₂. The redox check fits.
Lesson 48 of 51 · RXN-048
Redox in living systems
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Redox reactions run inside every living cell — including yours, right now.
The idea
Respiration is the reaction cells run for energy: glucose reacts with oxygen, producing carbon dioxide and water.
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O is the overall equation for respiration.
Respiration is an oxidation-reduction reaction — electrons transfer as the glucose reacts with oxygen.
Photosynthesis runs the overall change in reverse: plants use light energy to turn carbon dioxide and water into glucose and oxygen.
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ is the overall equation for photosynthesis.
Photosynthesis is an oxidation-reduction reaction too — electrons transfer as the glucose is built.
Two redox reactions in living systems
Reaction
Overall equation
Energy
Respiration
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O
releases energy for the cell
Photosynthesis
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
uses absorbed light energy
Respiration and photosynthesis are oxidation-reduction reactions running the same overall change in opposite directions.
Every cell that uses glucose with oxygen, and every leaf that makes glucose, is running a redox reaction.
Worked examples
Worked example 1. Which redox reaction do your muscle cells run when they use glucose and oxygen?
Step 1
Answer: respiration.
Worked example 2. A leaf in sunlight turns carbon dioxide and water into glucose and oxygen. To which reaction category does photosynthesis belong?
Step 1
Answer: it is an oxidation-reduction (redox) reaction.
You can now identify oxidation-reduction reactions in living systems.
Check your understanding
During a run, your muscle cells combine glucose with oxygen for energy. To which reaction category does this reaction — respiration — belong?
AAn oxidation-reduction reactioncorrect
BAn acid-base reaction
This option is wrong — you reached for a familiar category — no acid or base reacts here; electrons transfer as glucose reacts with oxygen, which is the redox signature.
CA precipitation reaction
This option is wrong — you reached for a solid-forming category — respiration forms carbon dioxide and water, not a precipitate, and it runs on electron transfer.
DNone of the three categories
This option is wrong — you may have assumed cell reactions sit outside chemistry's categories — respiration is a redox reaction: electrons transfer as glucose reacts with oxygen.
Respiration: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O. Electrons transfer as the glucose reacts with oxygen. Respiration is an oxidation-reduction reaction.
Check your understanding
Algae in a sunlit pond build glucose from carbon dioxide and water, releasing oxygen. To which reaction category does this reaction — photosynthesis — belong?
AAn oxidation-reduction reactioncorrect
BA precipitation reaction
This option is wrong — you reached for a solid-forming category — no solid precipitates from two solutions here; photosynthesis runs on electron transfer.
CAn acid-base reaction
This option is wrong — you reached for a familiar category — no acid or base reacts, and no ionic compound forms; the electron transfer makes photosynthesis a redox reaction.
DNone of the three categories
This option is wrong — you may have treated a light-powered reaction as its own kind — the light supplies energy, but the chemistry is electron transfer: a redox reaction.
Photosynthesis: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂. Electrons transfer as the glucose is built. Photosynthesis is an oxidation-reduction reaction.
Check your understanding
Which of these processes in a living system is a redox reaction?
AA cell reacting glucose with oxygen for energycorrect
BSweat evaporating from the skin
This option is wrong — you picked a state change — evaporation rearranges no atoms and transfers no electrons; it is a physical change.
CWater rising through a plant's stem
This option is wrong — you picked a movement of matter — the water is transported unchanged, with no reaction and no electron transfer.
DWarm blood transferring heat to cold fingertips
This option is wrong — you picked an energy flow — heat moving between body parts changes no substances and transfers no electrons.
A redox reaction needs a chemical reaction with electron transfer. Evaporation, transport, and heat flow change no substances. A cell reacting glucose with oxygen — respiration — is the redox reaction.
Lesson 49 of 51 · RXN-049
Redox in non-living systems
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Outside living things, redox reactions rust bridges, power flashlights, and burn fuels.
The idea
Rusting is an oxidation-reduction reaction: iron reacts with oxygen from the air, forming iron oxide — rust.
As iron rusts, the iron atoms give electrons to oxygen.
A battery's electricity comes from a redox reaction inside it — the transferred electrons flow out through the wire as electric current.
Every combustion is a redox reaction, so a gas stove's flame and a car engine burning fuel are redox reactions too.
Worked examples
Worked example 1. A steel garden gate slowly turns brown and flaky as its iron reacts with oxygen. To which reaction category does rusting belong?
Step 1
Answer: it is an oxidation-reduction (redox) reaction.
Worked example 2. What kind of reaction produces the electric current inside a battery?
Step 1
Answer: a redox (oxidation-reduction) reaction.
You can now identify oxidation-reduction reactions in non-living systems.
Check your understanding
A ship's iron hull slowly reacts with oxygen in seawater, forming crumbly iron oxide. To which reaction category does this change belong?
AAn oxidation-reduction reactioncorrect
BA precipitation reaction
This option is wrong — you matched the crumbly solid — a precipitation reaction needs two dissolved compounds mixing; here uncombined iron reacts with oxygen, giving electrons as it goes.
CAn acid-base reaction
This option is wrong — you reached for a familiar category — no acid or base reacts, and no water forms; the iron atoms giving electrons to oxygen is the redox signature.
DNone of the three categories
This option is wrong — you may have treated slow changes as non-reactions — rusting is a genuine reaction, and the electron transfer from iron to oxygen makes it redox.
Rusting: iron reacts with oxygen, forming iron oxide. The iron atoms give electrons to oxygen. Rusting is an oxidation-reduction reaction.
Check your understanding
A flashlight shines because current flows from its battery. What produces that current inside the battery?
AA redox reaction — the transferred electrons flow out as the current.correct
BElectricity stored at the factory, which drains out like water from a tank.
This option is wrong — you used the storage picture — a battery stores reactive chemicals, not electricity; the current is made by the redox reaction running inside.
CA precipitation reaction between the battery's chemicals.
This option is wrong — you named the wrong category — a precipitation reaction moves no electrons through a wire; the current comes from electron transfer, a redox reaction.
DThe metal case attracting electrons out of the surrounding air.
This option is wrong — you sourced the electrons outside the battery — the electrons come from the redox reaction between the chemicals inside.
A battery holds chemicals that react by transferring electrons. Those transferred electrons flow out through the wire as electric current. The current's source is a redox reaction.
Check your understanding
A furnace burns natural gas to heat a house: CH₄ + 2O₂ → CO₂ + 2H₂O. To which reaction category does this burning belong?
AAn oxidation-reduction reactioncorrect
BAn acid-base reaction
This option is wrong — you may have matched the water among the products — an acid-base reaction needs an acid and a base as reactants, and this is a combustion, which is always redox.
CA precipitation reaction
This option is wrong — you reached for a solid-forming category — the products are gases, and every combustion is a redox reaction.
DNone of the three categories
This option is wrong — you may have kept combustion as only a reaction TYPE — the type label stands, and by category every combustion is an oxidation-reduction reaction.
The equation is a combustion: a hydrocarbon reacting with O₂ to form CO₂ and H₂O. Every combustion is a redox reaction. The furnace runs an oxidation-reduction reaction.
Summary video — Acid-base, precipitation, and redox categories
Snap a disposable hand warmer and it pours out heat for hours as the iron powder inside reacts with oxygen. Stir citric acid into a baking-soda solution and the mixture turns cold in your hand. Two reactions — and energy moving in two opposite directions.
Chemists name the two directions, and the names do a lot of work in the rest of chemistry.
The idea
A reaction's 'surroundings' are everything around it — the container, the solution the chemicals sit in, the air, your hand.
An 'exothermic' reaction releases energy to its surroundings.
An 'endothermic' reaction absorbs energy from its surroundings.
A burning candle releases energy as heat and light, so burning a candle is exothermic.
Exothermic: energy released to the surroundings. Endothermic: energy absorbed from the surroundings.
The citric acid and baking soda reaction absorbs energy from its surroundings, so it is endothermic.
The prefixes name the energy's direction from the reaction's point of view: exo- means energy out, endo- means energy in.
The energy is not created or destroyed either way — it only moves between the reaction and the surroundings.
Worked examples
Worked example 1. A campfire pours heat and light into the night air. Is the burning exothermic or endothermic?
Step 1
Answer: exothermic — it releases energy to its surroundings.
Worked example 2. Photosynthesis runs only while the leaf absorbs light energy. Is photosynthesis exothermic or endothermic?
Step 1
Answer: endothermic — it absorbs energy from its surroundings.
You can now state that an exothermic reaction releases energy to its surroundings and an endothermic reaction absorbs energy from its surroundings.
Check your understanding
What does an exothermic reaction do with energy?
AIt releases energy to its surroundings.correct
BIt absorbs energy from its surroundings.
This option is wrong — you swapped the pair — absorbing energy is the endothermic direction; exo- means energy out.
CIt creates new energy inside the reacting mixture.
This option is wrong — you let the reaction make energy — energy is never created; an exothermic reaction moves existing energy out to the surroundings.
DIt keeps all its energy, moving none in either direction.
This option is wrong — you made the reaction energy-neutral — the exo- prefix names an energy flow outward, to the surroundings.
Exo- means energy out. An exothermic reaction releases energy to its surroundings. The energy moves; it is not created or destroyed.
Check your understanding
What does an endothermic reaction do with energy?
AIt absorbs energy from its surroundings.correct
BIt releases energy to its surroundings.
This option is wrong — you swapped the pair — releasing energy is the exothermic direction; endo- means energy in.
CIt destroys some of the surroundings' energy.
This option is wrong — you let energy vanish — energy is never destroyed; an endothermic reaction takes energy IN from the surroundings, which is why they are left with less.
DIt produces cold and spreads the cold outward.
This option is wrong — you treated cold as a substance — nothing called cold exists to spread; the reaction absorbs energy, and less energy around it is what feels cold.
Endo- means energy in. An endothermic reaction absorbs energy from its surroundings. The surroundings end up with less energy — that is the whole story of the cold.
Check your understanding
A glow stick releases energy as light while the chemicals inside react. Classify the reaction.
AExothermiccorrect
BEndothermic
This option is wrong — you swapped the directions — the reaction is giving energy OUT as light, and energy out is exothermic.
CBoth exothermic and endothermic
This option is wrong — you split the label — one reaction has one net energy direction, and this one releases energy.
DNeither exothermic nor endothermic
This option is wrong — you counted only heat as energy — light is energy too, and releasing it makes the reaction exothermic.
The reaction releases energy — as light rather than heat. Released energy in any form means the exothermic direction. The glow stick's reaction is exothermic.
Lesson 51 of 51 · RXN-051
Classifying by temperature change
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Exothermic and endothermic name the energy's direction — but no one hands you the direction in a lab. What you get is a thermometer and your own hands. This lesson turns a temperature change into the classification.
The idea
Watch the surroundings, not the reaction itself: the solution the chemicals sit in, the beaker, the air, your hand.
If the surroundings get hotter, the reaction released energy into them, so the reaction is exothermic.
If the surroundings get colder, the reaction absorbed energy from them, so the reaction is endothermic.
A beaker that feels hot means an exothermic reaction inside.
Reading the temperature change
Observation
Energy direction
Classification
surroundings get hotter
energy released into them
exothermic
surroundings get colder
energy absorbed from them
endothermic
The surroundings' temperature change points straight at the classification.
A mixture that turns cold has not made cold — the reaction pulled energy out of the surroundings, and less energy means a lower temperature.
Worked examples
Worked example 1. Two solutions are mixed in a flask. A thermometer in the mixture reads 21 °C before mixing and 38 °C two minutes after. Classify the reaction as exothermic or endothermic.
Step 1
The solution and flask are the reaction's surroundings, and their temperature rose from 21 °C to 38 °C.
Step 2
Hotter surroundings mean the reaction released energy into them.
Step 3
The reaction is exothermic.
Worked example 2. Baking soda is stirred into vinegar, and the liquid's temperature falls from 22 °C to 18 °C as the mixture fizzes. Classify the reaction.
Step 1
The liquid is the reaction's surroundings, and its temperature fell from 22 °C to 18 °C.
Step 2
Colder surroundings mean the reaction absorbed energy from them.
Step 3
The reaction is endothermic.
You can now classify a reaction as exothermic or endothermic from the direction of the observed temperature change of its surroundings.
Check your understanding
A self-heating meal pack lets water react with calcium oxide; within minutes the pack is hot enough to warm the food. Classify the reaction.
AExothermiccorrect
BEndothermic
This option is wrong — you swapped the directions — the pack (the surroundings) got HOTTER, so the reaction released energy into it.
CBoth exothermic and endothermic
This option is wrong — you split the label — a reaction has one net energy direction, and this one's surroundings only heated up.
DNeither exothermic nor endothermic
This option is wrong — you may have required a flame — no flame is needed; hotter surroundings alone show that energy was released.
Watch the surroundings: the pack and the food got hotter. Hotter surroundings mean the reaction released energy into them. The reaction is exothermic.
Check your understanding
A student mixes two solutions in a beaker, and the thermometer in the mixture falls from 23 °C to 11 °C. Classify the reaction.
AEndothermiccorrect
BExothermic
This option is wrong — you swapped the directions — the solution (the surroundings) got COLDER, so the reaction absorbed energy from it.
CNeither exothermic nor endothermic
This option is wrong — you may have expected the mixture itself not to count — at this level the solution the chemicals sit in IS part of the surroundings, and it cooled.
DBoth exothermic and endothermic
This option is wrong — you split the label — the temperature moved one way, so the energy moved one way: into the reaction.
Watch the surroundings: the solution's temperature fell from 23 °C to 11 °C. Colder surroundings mean the reaction absorbed energy from them. The reaction is endothermic.
Check your understanding
Inside a road flare, chemicals react with a brilliant light, and the flare's casing becomes very hot. Classify the reaction.
AExothermiccorrect
BEndothermic
This option is wrong — you swapped the directions — the casing (the surroundings) got hotter, so the reaction released energy into it.
CNeither exothermic nor endothermic
This option is wrong — you may have treated the light as using up the released energy — light AND the hot casing are both energy leaving the reaction.
DBoth exothermic and endothermic
This option is wrong — you counted light out and heat out as two directions — both flows go OUTWARD, one direction: exothermic.
Watch the surroundings: the casing became very hot. Hotter surroundings mean the reaction released energy into them — the light is released energy too. The flare's reaction is exothermic.