GCSE Particle Model — States of Matter, Density and Gas Pressure

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The particle model explains the properties of solids, liquids and gases in terms of how particles are arranged and how they move. It is the foundation for understanding density, changes of state, thermal energy, and gas behaviour. This guide covers the full topic with all the calculations that accompany it.

The Three States of Matter

All matter consists of particles (atoms, molecules or ions). The state of a substance depends on how these particles are arranged and how much energy they have.

Solid: Particles are closely packed in a regular arrangement. They vibrate about fixed positions but cannot move past each other. Solids have a fixed shape and volume. They cannot be compressed.
Liquid: Particles are close together but in a random arrangement. They can move past each other — liquids flow and take the shape of their container. Volume is fixed but shape is not. Very difficult to compress.
Gas: Particles are far apart, moving rapidly and randomly in all directions. Gases fill any container they are in. Can be compressed easily. Very low density compared to solids and liquids.

Changes of State

When a substance changes state, energy is transferred to or from the particles but the temperature does not change during the change of state — all the energy goes into breaking or forming bonds between particles, not into increasing their kinetic energy.

Melting (solid → liquid): energy absorbed, bonds between particles weakened
Boiling/evaporation (liquid → gas): energy absorbed, bonds between particles broken
Condensing (gas → liquid): energy released
Freezing (liquid → solid): energy released

During a change of state, temperature stays constant even though energy is being transferred. On a heating curve graph, the flat sections represent changes of state — melting and boiling. The temperature only rises during the sections where the state is not changing.

Density

Density is the mass per unit volume of a substance. Materials with particles closely packed have high density. Gases have very low density because particles are spread far apart.

Density: ρ = m/V
ρ = density (kg/m³), m = mass (kg), V = volume (m³)

To find the density of an irregular solid: measure its mass using a balance. Find its volume by water displacement — submerge it in a measuring cylinder of water and record the increase in volume. Then ρ = m/V.

Specific Latent Heat

Specific latent heat is the energy needed to change the state of 1 kg of a substance without changing its temperature. There are two types:

Specific latent heat of fusion (L_f): Energy to melt (or freeze) 1 kg
Specific latent heat of vaporisation (L_v): Energy to boil (or condense) 1 kg

Energy for change of state: Q = mL
Q = energy (J), m = mass (kg), L = specific latent heat (J/kg)

The specific latent heat of vaporisation is always larger than that of fusion for the same substance — boiling requires breaking all bonds between particles, whereas melting only partially disrupts them.

Gas Pressure and the Particle Model

Gas pressure is caused by gas particles colliding with the walls of their container. Each collision exerts a tiny force on the wall. The total pressure is the combined effect of billions of these collisions per second.

Factors Affecting Gas Pressure

Temperature: Higher temperature → particles have more kinetic energy → move faster → collide more frequently and with greater force → higher pressure (if volume is fixed).
Volume: Smaller volume → particles are more confined → collide with walls more frequently → higher pressure (if temperature is fixed).
Number of particles: More particles → more collisions → higher pressure.

Boyle's Law (Higher Tier)

For a fixed mass of gas at constant temperature, pressure and volume are inversely proportional: pV = constant. If you halve the volume, the pressure doubles.

Boyle's Law: p₁V₁ = p₂V₂

Explaining Pressure Changes Using the Particle Model

Every exam answer about gas pressure must reference: (1) particles colliding with the walls of the container, (2) frequency of collisions, (3) force of collisions. A complete answer for why heating a gas increases pressure: "The particles gain kinetic energy and move faster. They collide with the walls more frequently and with greater force, so pressure increases." Missing any of these three elements loses marks.

The AQA particle model specification is at the AQA GCSE Physics specification page.

Practise Particle Model Questions

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