States of Matter and Changes of State — AQA GCSE Chemistry
The particle model explains the three states of matter and the changes between them.
The three states
| State | Arrangement | Movement | Shape/volume |
|---|---|---|---|
| Solid | regular, packed | vibrate in place | fixed shape and volume |
| Liquid | close, irregular | move past each other | fixed volume, takes container shape |
| Gas | far apart | fast, random | fills the container |
The stronger the forces between particles, the higher the melting and boiling points.
Changes of state
- Melting (solid → liquid) and freezing (liquid → solid) at the melting point.
- Boiling/evaporating (liquid → gas) and condensing (gas → liquid) at the boiling point.
- Sublimation (solid → gas directly).
Changes of state are physical changes — they are reversible and no new substance is formed; the amount of substance (mass) is conserved.
State symbols
In equations: (s) solid, (l) liquid, (g) gas, (aq) aqueous (dissolved in water).
Limitations of the simple model
The particle model assumes the particles are solid spheres with no forces between them and that they are not themselves made of smaller parts — which is not entirely accurate.
Exam tips
- Describe each state in terms of arrangement, movement and forces.
- Changes of state are physical (reversible, no new substance, mass conserved).
- Learn the state symbols (s, l, g, aq).
- Know the limitations of the simple particle model.