The particle model in chemistry
States of matter
Three states are explained by the particle (kinetic) model: small particles in constant motion, with arrangement and forces giving each state its bulk properties.
| State | Spacing | Arrangement | Movement |
|---|---|---|---|
| Solid | Touching | Regular lattice | Vibrate in place |
| Liquid | Touching | Random | Slide past one another |
| Gas | Far apart | Random | Fast, in all directions |
Limitations of the simple model
The simple "spheres in a box" picture is useful but has weaknesses you must be able to identify:
- Treats particles as identical, solid spheres (real molecules have different shapes/sizes).
- Ignores forces between particles.
- Ignores the space inside an atom (electrons, nucleus).
- Doesn't show that particles in a liquid are still attracted strongly enough to keep a fixed volume.
Changes of state
solid ⇌ liquid (melting / freezing) · liquid ⇌ gas (boiling / condensing) · solid ⇌ gas (sublimation, e.g. iodine, dry ice).
These are physical changes — no new substance forms and the change is reversible. Mass is conserved.
Energy and changes of state
Heating a solid at constant rate gives a heating curve with two flat plateaus — at the melting and boiling points — where energy goes into breaking inter-particle forces (potential energy) instead of raising temperature.
Density (links to physics)
ρ = m / V. Solids are densest because particles are touching; gases are least dense because particles are far apart.
Predicting state at a given temperature
If T < melting point → solid. If melting point < T < boiling point → liquid. If T > boiling point → gas. e.g. ethanol melts at −114°C and boils at +78°C, so at 25°C it is a liquid.
OCR exam tip
Two-mark "state and explain" questions need both halves: name the state AND link to particle behaviour. e.g. "It is a gas because the particles are far apart and move randomly with weak forces between them."
AI-generated · claude-opus-4-7 · v3-ocr-combined-science-leaves