Solids, Liquids and Gases

Everything is made of tiny particles that are always moving. In a solid they vibrate in fixed positions; in a liquid they are close together but slide past one another; in a gas they are far apart and move quickly in all directions. Heating gives particles more energy; at melting and boiling points that energy is used to overcome the forces between particles, so the temperature stays constant.

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Section 1 of 14

Learning objectives

Describe the arrangement, separation and motion of particles in solids, liquids and gases.
Name the changes of state and explain them with the particle model.
Interpret heating and cooling curves.
Explain how temperature and pressure affect the volume of a gas.

Pause and think

A kettle keeps heating boiling water, yet the thermometer stays at $100\,^\circ\mathrm{C}$ . Where is the energy going?

Section 2 of 14

Core idea

Start hereThe stubborn thermometer

Put a thermometer in a pan of melting ice and turn up the heat. The ice keeps melting, but the thermometer stays stuck at $0\,^\circ\mathrm{C}$ until the very last piece has gone. Where is all that energy going?

The big idea

All matter is made of particles that are always moving. The state depends on how strongly the particles attract each other compared with how much energy they have. During a change of state, energy goes into overcoming (or forming) attractions, so the temperature does not change.

1. The particle model

Solid: particles touching, regular pattern, vibrate about fixed positions. Fixed shape and volume.
Liquid: particles touching, random arrangement, slide past each other. Fixed volume, flows.
Gas: particles far apart, random, moving fast in all directions. Fills its container, easily compressed.

Changes of state

Drawing diagram…

Heating moves right; cooling moves left.

2. Heating and cooling curves

Worked exampleReading a heating curve

A solid is heated steadily. Its temperature rises to $44\,^\circ\mathrm{C}$ , stays there for 4 minutes, then rises to $280\,^\circ\mathrm{C}$ , stays there for 10 minutes, then rises again.

Goal: State the melting and boiling points and the state during each flat section.
Strategy: Flat sections are changes of state; the first is melting, the second is boiling.

$\text{m.p.}=44\,^\circ\mathrm{C}$
First flat section: solid and liquid together.
$\text{b.p.}=280\,^\circ\mathrm{C}$
Second flat section: liquid and gas together.

AnswerMelts at

44\,^\circ\mathrm{C}

, boils at

280\,^\circ\mathrm{C}

✓ CheckThe boiling section is longer: separating particles completely needs more energy than loosening them ✓.

Key takeawayEvaporation vs boiling Evaporation happens only at the surface, at any temperature. Boiling happens throughout the liquid, only at the boiling point.

3. Gases, temperature and pressure

Heating a gas gives its particles more kinetic energy. They hit the container walls harder and more often; if the pressure is kept constant, the gas expands. Squeezing a gas at constant temperature pushes the particles closer together, so its volume falls.

Common misconceptionParticles do not expand When a substance expands on heating, the particles move further apart. The particles themselves stay the same size.

Pause and think

Why can you smell a hot curry from further away than a cold one?

Summary

Describe arrangement, separation and motion.
Flat sections on curves = changes of state.
Gas volume rises with $T$ , falls with $p$ .

Melting point: Temperature at which a solid turns into a liquid.
Boiling point: Temperature at which a liquid boils throughout.
Kinetic particle theory: Matter is made of moving particles.

HistoryDemocritus. Around

400

BC the Greek philosopher Democritus argued that matter is made of tiny indivisible pieces he called atomos. It took over 2000 years for experiments to prove him right.

Fun factGlass is not a slow liquid. A popular myth says old church windows are thicker at the bottom because glass flows. It's false: glass is an amorphous solid, and the windows were simply made unevenly.

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