Particle Model of Matter: States and Changes of State — AQA GCSE Physics
The particle model explains the properties of solids, liquids and gases and the changes between them.
The three states
| State | Arrangement | Movement |
|---|---|---|
| Solid | regular, closely packed | vibrate about fixed positions |
| Liquid | close together, irregular | move past each other |
| Gas | far apart | fast, random motion |
The stronger the forces between particles, the higher the melting and boiling points.
Changes of state
Changes of state are physical changes (not chemical) — the substance keeps the same mass (mass is conserved) and the change is reversible, with no new substance formed.
- Melting/freezing at the melting point.
- Boiling/evaporating/condensing at the boiling point.
- Sublimation (solid ↔ gas directly).
Energy and changes of state
When a substance changes state, energy is transferred but the temperature stays constant, because the energy is used to overcome the forces between particles, not to raise their kinetic energy.
Density and states
Solids are usually the most dense (particles closest together) and gases the least dense (particles far apart).
Exam tips
- Describe each state by arrangement, movement and forces.
- Changes of state are physical (mass conserved, reversible, no new substance).
- Temperature stays constant during a change of state.
- Link density to how closely packed the particles are.