Kinetic theory
The kinetic model treats all matter as made of particles in continuous motion. The amount of motion (energy) and the spacing of particles determines the state.
States of matter
| State | Spacing | Arrangement | Motion |
|---|---|---|---|
| Solid | Very close | Regular lattice | Vibrate about fixed positions |
| Liquid | Close | Disordered | Slide past each other |
| Gas | Far apart | Random | Fast straight-line motion until collisions |
Changes of state
melting (solid → liquid), freezing (liquid → solid), boiling/evaporation (liquid → gas), condensation (gas → liquid), sublimation (solid → gas, e.g. iodine, dry ice).
Specific heat capacity (c)
Energy needed to raise the temperature of 1 kg of a substance by 1 °C.
E = m × c × ΔT (J = kg × J/kg/°C × °C)
For water, c = 4200 J/kg/°C — this is unusually high, which is why water is used as a coolant and as a heat store.
Latent heat (L)
Energy needed to change the state of a substance, without any temperature change.
E = m × L (J = kg × J/kg)
- Latent heat of fusion — melting/freezing.
- Latent heat of vaporisation — boiling/condensation.
During a change of state, the energy supplied breaks intermolecular forces (or is released when they form), so the temperature plateaus on a heating curve.
Reading a heating curve
Steps in a typical heating curve for water from −20 °C to 120 °C:
- Ice warms from −20 °C to 0 °C (slope, c_ice).
- Ice melts at 0 °C — flat line (latent heat of fusion).
- Water warms from 0 °C to 100 °C (slope, c_water).
- Water boils at 100 °C — flat line (latent heat of vaporisation).
- Steam warms above 100 °C.
CCEA tip
When you see a heating curve question, the marks are usually about identifying which segments are sloped vs flat and naming the energy involved (specific heat for slopes, latent heat for plateaus). One sentence per segment scores efficiently.
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