A change of state occurs when a substance changes between solid, liquid and gas. During a change of state, the temperature remains constant even though energy is being supplied or removed.
The changes of state
| Change | From | To | Energy |
|---|---|---|---|
| Melting | Solid | Liquid | Energy absorbed |
| Boiling/evaporation | Liquid | Gas | Energy absorbed |
| Condensation | Gas | Liquid | Energy released |
| Freezing | Liquid | Solid | Energy released |
| Sublimation | Solid | Gas | Energy absorbed |
Why temperature stays constant during a change of state
During melting or boiling, the energy supplied is used to break the bonds between particles, not to increase their kinetic energy. Since temperature depends on the average kinetic energy of the particles, the temperature does not change until the change of state is complete.
Heating curve
A heating curve plots temperature against time for a substance being heated at a constant rate:
- Solid phase: temperature rises as particles vibrate faster.
- Melting: temperature stays constant at the melting point; energy breaks bonds.
- Liquid phase: temperature rises as particles move faster.
- Boiling: temperature stays constant at the boiling point; energy breaks bonds.
- Gas phase: temperature rises as particles move faster.
Evaporation vs boiling
| Evaporation | Boiling | |
|---|---|---|
| Temperature | Below boiling point | At boiling point |
| Where it occurs | Surface only | Throughout the liquid |
| Bubble formation | No | Yes |
| Rate | Gradual | Rapid |
Common errors and how to correct them
Saying “particles melt” or “particles boil.” Particles do not change state individually. The substance changes state; the particles gain energy and change arrangement.
Stating temperature rises during melting. Temperature is constant during a change of state. The energy input goes into breaking intermolecular bonds, not increasing kinetic energy.
How to apply this in an exam
When interpreting a heating curve, identify the flat sections as changes of state (melting or boiling) and the sloped sections as temperature increases within a single state.
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