Ask two questions about any sample: does it keep its own shape, and does it keep its own volume?

A solid keeps both — its shape and its volume stay the same in any container and on any surface.

A liquid keeps its own volume but not its own shape — it flows until it matches the shape of its container.

A gas keeps neither — it spreads out to take both the shape and the entire volume of its container.

Apply the two questions to the material itself, not to a heap of pieces.

One grain of sand keeps its own shape and its own volume anywhere you put it, so sand is a solid — a jarful of sand is just many small solids.

In a solid, the particles are packed tightly together in fixed positions, usually in a regular pattern.

Solid particles cannot swap places — each one only vibrates in place.

In a liquid, the particles are still touching, but they sit in no fixed positions and no pattern.

Liquid particles slide past one another.

In a gas, the particles are far apart, with empty space between them.

Gas particles move fast in all directions, spreading through all the space available.

A solid keeps its own shape because its particles are locked in fixed positions.

Each particle of a solid can only vibrate in place, so the whole solid holds one shape.

A liquid flows because its particles can slide past one another.

When you pour a liquid into a container, its sliding particles rearrange until the liquid matches the container's shape.

A gas's particles are far apart, with empty space between them.

Squeezing a gas pushes its particles closer together into that empty space, so the same particles fit in a smaller volume.

Squeezing a sample into a smaller volume is called 'compressing' it.

A solid's or a liquid's particles are already touching, so there is no empty space to push them into.

That is why a gas can be compressed while a solid or a liquid cannot.

Compressing a gas changes nothing about the particles themselves — the same number of identical particles simply sit closer together.

To read a particle diagram, ask two questions about the circles: are they touching, and are they arranged in a pattern?

Circles touching and arranged in a regular pattern show a solid.

Circles touching but jumbled, filling the bottom of the box, show a liquid.

Circles far apart, spread through the whole box, show a gas.

The circles' spacing and arrangement carry the answer — the number of circles does not.

Draw every particle as a circle of the same size, and draw the container as a box.

For a solid: circles touching, arranged in a regular repeating pattern, at least two rows deep.

For a liquid: circles touching but jumbled, filling the bottom of the box with a roughly level top.

For a gas: a few circles far apart, spread through the whole box — corners and top included.

Three drawing mistakes to avoid: gaps between a liquid's circles, a gas's circles bunched in one part of the box, and circles that change size.

On paper, draw a particle diagram of the milk in a half-full glass. Draw the glass as a box. When your drawing is finished, reveal the model answer and check your drawing against the checklist.

The change from solid to liquid is called 'melting' — an ice cube melts into water.

The change from liquid to solid is called 'freezing' — water in the tray freezes into ice.

The change from liquid to gas is called 'evaporation' — a puddle evaporates into water vapor.

The change from gas to liquid is called 'condensation' — water vapor condenses into the fog on a cold mirror.

The change from solid straight to gas, skipping the liquid state, is called 'sublimation' — dry ice, which is solid carbon dioxide, sublimes into gas without ever being wet.

The change from gas straight to solid, skipping the liquid state, is called 'deposition' — water vapor deposits as frost on a freezing-cold window.

The six names form three opposite pairs: melting and freezing, evaporation and condensation, sublimation and deposition.

Heating a solid makes its particles vibrate faster.

When the particles vibrate hard enough to break out of their fixed positions and start sliding past one another, the solid melts.

Heating a liquid makes its particles move faster still, until they fly apart from one another and spread out — the liquid evaporates.

Cooling runs each change in reverse.

In condensation, gas particles slow down until they crowd back into touching, sliding contact.

In freezing, liquid particles slow down until they lock into fixed positions.

Every state change works this way because the particles themselves do not change — only their spacing and movement change.

Some matter is so hot and so energetic that it no longer behaves like any of the three everyday states.

A gas heated until its particles become electrically charged has turned into a fourth state of matter, called 'plasma'.

Plasma glows — a lightning bolt, the Sun and the stars, and the glowing gas inside a lit neon sign are all plasma.

Almost all the ordinary matter in the universe is plasma, because the stars are made of it.

What 'electrically charged' means for a particle is a story for the atomic-structure unit — for now, charged particles are what make plasma different from a gas.

Zoom in on the first glass, and every particle is the same: each one is a water molecule.

Zoom in on the salt water, and two kinds of particles are mingled together: water molecules and salt particles.

Matter made of only one kind of particle is called a 'pure substance'.

Matter containing more than one kind of particle mingled together is called a 'mixture'.

The helium in a party balloon is a pure substance — every particle is a helium atom.

Air is a mixture — nitrogen particles, oxygen particles, and several other kinds, all mingled together.

'Pure' here means one kind of particle only — it does not mean clean, natural, or safe to drink.

A bar of pure gold contains only gold atoms — one kind of atom and nothing else.

A pure substance made of only one kind of atom is called an 'element'.

Helium, gold, copper, iron, oxygen, and nitrogen are all elements.

The periodic table you have seen on classroom walls is the complete list of the elements.

An element's atoms may travel joined in pairs — oxygen gas is molecules of two oxygen atoms joined together — but with only one kind of atom, it is still an element.

One particle of pure water contains two hydrogen atoms joined to one oxygen atom.

Every particle of pure water is built exactly the same way — always two hydrogen atoms and one oxygen atom, never one, never three.

A pure substance whose particles each contain atoms of two or more different elements chemically joined together is called a 'compound'.

In a compound, the ratio of atoms is fixed: every particle of the compound is built to the same recipe.

Carbon dioxide is a compound — each of its particles is one carbon atom joined to two oxygen atoms.

Table salt is a compound — sodium and chlorine atoms joined in a fixed one-to-one ratio.

A compound is one pure substance, not a mixture: its different atoms are chemically joined into a single kind of particle, not mingled loosely.

Chemically joining atoms makes a new kind of particle.

A new kind of particle means a new substance, and a new substance has its own properties.

A particle of table salt is not a bit of sodium plus a bit of chlorine — it is a different particle, so salt behaves like salt and like nothing else.

The elements' old properties belong to the elements' own particles, and those particles are gone once the atoms are joined into the compound.

The same holds for every compound: water is nothing like hydrogen, a gas that burns explosively, or oxygen, a gas that feeds flames — water puts flames out.

In a particle diagram, different kinds of atoms are drawn as circles of different sizes or shades.

Circles drawn touching-and-fused belong to one joined particle; particles with clear space between them are separate.

Ask two questions: how many kinds of particles are in the box, and what is each particle built from?

One kind of particle, each built from one kind of atom — alone or in identical joined pairs — shows an element.

One kind of particle, each built from two or more different kinds of atoms joined together, shows a compound.

More than one kind of particle mingled loosely in the box shows a mixture.

Before you draw, decide two things: how many kinds of circles the diagram needs, and which circles are joined.

Use circles of different sizes or shading for different kinds of atoms, exactly as in the diagrams you've already read.

For an element, draw only one kind of circle.

Every particle of the element must look the same — single circles, or identical joined groups of that one circle.

For a compound, draw one kind of particle, with every particle built from the same two or more different circles joined together.

No lone circles may be left over in a compound — every atom belongs to an identical joined particle.

For a mixture, draw two or more different kinds of particles mingled through the same box, with the different kinds not joined to each other.

The state rules you've already used still apply — tightly packed rows for a solid, touching circles in no fixed pattern gathered at the bottom for a liquid, widely spaced circles filling the whole box for a gas.

On paper, draw a particle diagram of the element neon — a gas whose particles are single atoms — in a sealed box. Draw at least six particles, then select Continue to compare your drawing with the model answer.

Salt water is the same throughout — every part of it has the same makeup and the same properties.

A mixture that is the same throughout is called a 'homogeneous mixture'.

Air with no dust or smoke is also homogeneous — every breath in a room has the same makeup.

Vegetable soup keeps visibly different parts — one region is broth, another is carrot.

A mixture whose parts remain visibly different is called a 'heterogeneous mixture'.

Muddy water is also heterogeneous — solid specks drift through the liquid as separate visible parts.

In the names, 'homo-' means same and 'hetero-' means different.

The two names answer one question about any mixture: is it the same throughout, or do its parts remain visibly different?

To classify a mixture, ask one question: does every part of it have the same makeup, or can you see or pick out different parts?

If every part is the same — every sip, every spoonful, every sample — the mixture is homogeneous.

If different parts remain visible — layers, pieces, specks, or regions — the mixture is heterogeneous.

Judge the mixture as a whole, not its ingredient list — a homogeneous mixture still contains more than one kind of particle.

Counting ingredients does not decide the answer; a mixture of many ingredients is still homogeneous when it is the same throughout.

Solids and gases classify the same way as liquids — a metal blend with its atoms evenly mingled is homogeneous, and dust swirling in air is heterogeneous.

First question: does the sample contain one kind of particle, or more than one kind mingled together?

One kind of particle means a pure substance; more than one kind means a mixture.

For a pure substance, ask the second question: are its particles made of one kind of atom, or of two or more kinds joined together?

One kind of atom means an element; two or more kinds joined in every particle means a compound.

For a mixture, ask the second question: is it the same throughout, or do its parts remain visibly different?

The same throughout means a homogeneous mixture; visibly different parts mean a heterogeneous mixture.

Every sample of matter lands in exactly one of the four classes.

Some properties can be observed or measured while the substance stays itself — checking them turns the substance into nothing new.

A property like that is called a 'physical property'.

Color, hardness, and the state at room temperature are physical properties.

The temperature at which a solid melts is called its 'melting point', and the temperature at which a liquid boils is called its 'boiling point'.

Melting point and boiling point are physical properties — the melted or boiled substance is still the same substance.

Other properties describe how a substance can turn into a different substance.

A property like that is called a 'chemical property'.

Flammability — the ability to burn — is a chemical property, because burning turns a substance into new substances.

The ability of iron to rust is also a chemical property, because rust is a new substance formed from the iron.

To classify a property, ask one question: can it be observed or measured while the substance stays the same substance?

If yes, the property is physical.

If the property only shows itself as the substance turns into a different substance, the property is chemical.

The tools involved do not decide the answer — thermometers, flames, and instruments can appear in checks of either kind.

State changes do not make a property chemical — a substance that melts, boils, or freezes is still the same substance.

In some changes, the substance keeps its identity — it is the same substance before and after, in a new shape, size, or state.

A change like that is called a 'physical change'.

Crushing, tearing, melting, freezing, boiling, and dissolving are physical changes.

In other changes, one or more new substances form — the starting substance stops being itself.

A change like that is called a 'chemical change'.

Burning and rusting are chemical changes — the materials left behind are new substances.

One question separates the two kinds: did a new substance form?

During a state change, every particle keeps being exactly the particle it was.

Melting ice loosens the water particles from their fixed positions so they can slide past one another — but each particle is still a water particle.

Boiling spreads the water particles far apart so they fly freely — but each particle is still a water particle.

State changes are physical changes because the particles themselves do not change — only their spacing and movement change.

Dissolving works the same way: sugar particles spread out and mingle among water particles, and both kinds stay exactly what they were.

The sweet taste of sugar water is the unchanged sugar particles at work.

A chemical change is different — new substances form, so the particles present afterward are no longer the particles you started with.

A color change is a sign — a sliced apple slowly turning brown.

A new gas is a sign — fizzing or bubbles appearing in a liquid that is not being boiled.

A new solid is a sign — a solid appearing when two liquids are mixed.

A temperature change is a sign — a mixture growing hot or cold on its own, with no outside heating or cooling.

Given-off light is a sign — a material glowing or flaming.

Each sign says 'a chemical change MAY have happened' — the signs are clues, not proof.

A sign is a clue, not proof — bubbles rise through boiling water, yet no new substance is forming.

The gas in those bubbles is water vapor: still water, only in the gas state.

To judge a sign, ask the deciding question: did a new substance actually form?

If the 'new' material is the same substance in a new state or a new place, the change is physical and the sign fooled you.

If the material really is a different substance from everything you started with, the sign is confirmed.

Outside causes can fake a sign too — a mixture warmed by a stove shows a temperature change without any chemical change.

To classify a described change, ask the one deciding question: did a new substance form?

Judge by the evidence, not the drama — a slow, quiet change can be chemical, and a sudden, violent one can be physical.

Check each sign before trusting it — a gas that is just the boiled starting substance, or warmth from an outside heater, proves nothing.

Changes of shape, size, state, or mixing leave the substance itself, so they are physical.

Evidence that a genuinely different substance appeared makes the change chemical.

An observation that describes a quality in words — a color, a smell, a texture, a behavior — is called 'qualitative data'.

'The liquid turned cloudy' is qualitative: it records what was seen, with no number.

An observation that records a measured number or a count is called 'quantitative data'.

'The water reached 38.5 °C' is quantitative: it records a measured number.

The test is the record itself — does the observation contain a measured number or count?

What could have been measured does not matter: 'the beaker felt hot' is qualitative, even though a thermometer could have given a number.

Scientists everywhere share one set of measurement units, called the 'International System of Units' — 'SI' for short.

For mass, chemists record grams (g), and kilograms (kg) for larger amounts.

For volume, chemists record liters (L) and milliliters (mL), and cubic centimeters (cm³) work for solids and liquids alike.

One milliliter is exactly one cubic centimeter — 1 mL = 1 cm³ — so a volume keeps the same number in either unit.

For temperature, chemists record degrees Celsius (°C) in everyday lab work.

The SI base unit for temperature is the 'kelvin' (K) — for now only its name matters; the course returns to it with gases.

A word attached to the front of a unit to resize it is called a 'unit prefix'.

'kilo-' means one thousand: one kilogram is one thousand grams.

'centi-' means one hundredth: one centimeter is one hundredth of a meter.

'milli-' means one thousandth: one milliliter is one thousandth of a liter.

'nano-' means one billionth: one nanometer is one billionth of a meter.

A prefix keeps its meaning on any unit — a milligram is one thousandth of a gram, just as a milliliter is one thousandth of a liter.

That medicine label can read '5 mL' instead of '0.005 L' — the prefix carries the zeros.

To convert a measurement to its base unit, replace the prefix with the number it means.

First write what one prefixed unit equals in the base unit.

milli- means one thousandth, so 1 mL = 0.001 L.

Then multiply the measured number by that meaning.

The converted value is the same measurement written in a different unit — nothing about the sample changed.

The prefix meanings you've already seen work in both directions.

milli- means one thousandth, so 1 mL = 0.001 L — read the other way, one liter holds 1000 mL.

To convert a base-unit value to a prefixed unit, find how many prefixed units fit in it.

Multiply the base-unit value by how many prefixed units make up one base unit.

One liter holds 1000 mL, so multiply a value in liters by 1000 to write it in milliliters.

One thousand grams make a kilogram, so only 0.001 kilograms fits in one gram — multiplying by 0.001 is the same as dividing by 1000, so divide a value in grams by 1000 to write it in kilograms.

The converted value is still the same measurement — only the unit changed.

A tall container with volume marks up its side is called a 'graduated cylinder' — it is the standard tool for measuring liquid volume.

Water in a graduated cylinder does not sit flat.

The surface climbs slightly where the water touches the glass walls, so the surface dips into a curve.

That curved surface is called the 'meniscus'.

Read the volume at the bottom of the meniscus — the lowest point of the curve.

Keep your eye level with the water surface when you read.

Looking down from above or up from below shifts which mark the meniscus appears to touch, so the reading comes out wrong.

The digits a measurement vouches for — every certain digit plus the one estimated digit at the end — are called its 'significant figures'.

All nonzero digits are significant, so 36.5 mL has three significant figures.

Zeros sitting between nonzero digits are significant, so 205 g has three significant figures.

Zeros in front of the first nonzero digit are not significant — they only place the decimal point.

So counting in 0.0250 g starts at the 2.

Zeros at the end of a number that has a decimal point are significant — the instrument really did vouch for them.

So 0.0250 g has three significant figures: the 2, the 5, and the final 0.

When you multiply or divide measurements, count the significant figures in each measurement first.

Round the calculator's result to the same number of significant figures as the measurement with the fewest.

The least precise measurement sets the precision of the result.

A result can never be more trustworthy than the shakiest measurement that went into it.

The lead cube packs far more mass into the same 10.0 cm³ of space than the foam cube does.

How much mass a material packs into each unit of volume is called its 'density'.

Lead has a high density — each cubic centimeter of lead carries a lot of mass.

Plastic foam has a low density — each cubic centimeter of foam carries very little mass.

Density is a physical property — you can measure it without turning the material into anything new.

Hold the volume the same: here are two cubes, each exactly 5.0 cm³.

The copper cube has a mass of 44.5 g.

The aluminum cube has a mass of 13.5 g.

The copper cube packs more mass into the same 5.0 cm³, so copper is the denser material.

When two samples have equal volumes, the one with the greater mass has the greater density.

Hold the mass the same: here are two solid blocks, each exactly 100 g.

Block A takes up 20.0 cm³.

Block B takes up 50.0 cm³.

Block A squeezes the same 100 g into less space, so block A is made of the denser material.

When two samples have equal masses, the one with the smaller volume has the greater density.

Density is the mass divided by the volume.

D = m / V

D stands for density, measured in grams per cubic centimeter (g/cm³).

m stands for mass, measured in grams.

V stands for volume, measured in cubic centimeters.

Dividing the mass by the volume shares the mass out over every cubic centimeter, so D is the mass packed into each one — exactly what density means.

When the volume is held the same, a greater mass makes D greater.

When the mass is held the same, a smaller volume makes D greater.

Write down the mass and the volume from the question.

Write down the equation, D = m/V.

Substitute the values and calculate.

The result carries the unit g/cm³.

Round the result to the significant figures of the least precise measurement, as you've already practiced.

Use the density equation you've already seen: D = m/V.

The question gives D and V and asks for m, so rearrange the equation.

Make m the subject by multiplying both sides by V, giving m = D × V.

Then substitute and calculate — the mass comes out in grams.

It is one equation, rearranged when needed, not a second equation to memorize.

Use the density equation you've already seen: D = m/V.

The question gives D and m and asks for V, so rearrange the equation.

Make V the subject: multiply both sides by V, then divide both sides by D, giving V = m / D.

Then substitute and calculate — the volume comes out in cubic centimeters.

It is still the one density equation, rearranged for the quantity you need.

Partly fill a graduated cylinder with water and read the level at the bottom of the meniscus.

Lower the solid gently until it is fully underwater.

The solid pushes the water aside, so the water level rises.

The rise in the level equals the volume of the solid — the water moved up exactly as much space as the solid took.

Finding a volume this way is called 'water displacement'.

Volume of the solid = final level − starting level.

The levels are read in milliliters, and one milliliter is one cubic centimeter, so the volume comes out in cm³.