IGCSE §5: Solids, Liquids and Gases · IGCSE
Changes of State
Revision notes on Changes of State for the OCR A-level Physics specification (H556). Free to read, with 4 practice questions in the app.
| Change | From | To |
|---|---|---|
| Melting | Solid | Liquid |
| Freezing (solidifying) | Liquid | Solid |
| Boiling / evaporating | Liquid | Gas |
| Condensing | Gas | Liquid |
| Sublimation | Solid | Gas directly |
A change of state is physical, not chemical. No new substance is made, the mass is unchanged, and the change can be reversed. Only the arrangement, spacing and motion of the particles are different.
Why the temperature stays constant during a change of state. While ice is melting, energy is still being supplied, but the thermometer does not move. The energy is being used to overcome the forces between particles and separate them, not to increase their kinetic energy. Temperature is a measure of the average kinetic energy of the particles, so while that energy is going into breaking bonds rather than speeding particles up, the temperature holds steady.
On a heating graph this appears as a horizontal plateau at the melting point and another at the boiling point.
Evaporation and boiling are not the same thing:
| Evaporation | Boiling | |
|---|---|---|
| Temperature | Any temperature | Only at the boiling point |
| Where | Surface only | Throughout the liquid |
| Speed | Gradual | Rapid, with bubbles |
Why evaporation cools what is left behind. The particles in a liquid have a range of speeds. Only the fastest ones have enough energy to escape from the surface. When they leave, the average kinetic energy of the particles remaining is lower — and a lower average kinetic energy is a lower temperature. This is why sweating cools you, and why a wet towel feels cold.
What makes evaporation faster: a higher temperature, a larger surface area, a draught carrying the escaped molecules away, and lower humidity.
Example: a beaker of ice at −10 °C is heated steadily. The temperature climbs to 0 °C, then holds there while all the ice melts, then climbs again to 100 °C, then holds while the water boils away. The two flat sections are where the energy is being spent separating particles rather than speeding them up.
4 questions on Changes of State
Reading is not revising. This topic has 4 questions in the app — multiple choice, calculations, and written answers marked against the mark scheme, point by point. Miss one and it comes back later with its options reshuffled, so you have to know it rather than remember where the answer sat.
- Every topic of Edexcel International GCSE Physics (4PH1)
- Flashcards and every equation, linked to the topic it comes from
- Full mock papers in the real paper structure
20 minutes a day is free. No card, nothing to cancel — and the paid version never renews itself either.
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