# Use the kinetic theory of matter to explain what happens when a solid is heated until it melts and then continues to be heated until it becomes a gas.

> WJEC GCSE Physics (Wales) — 1.8 Kinetic theory · Explain · 4 marks

## Mark scheme (4 marks)

1. When a solid is heated, the particles gain (kinetic) energy and vibrate more (vigorously) about their fixed positions
2. At the melting point, particles have enough energy to break free from their fixed positions / overcome the forces holding them in place, so they can move/flow (liquid state)
3. Continued heating gives the liquid particles more (kinetic) energy so they move faster / more quickly
4. At the boiling point, particles have enough energy to completely overcome the forces between them and escape / move freely and independently in all directions (gas state)

## Key terms

- [kinetic theory](https://www.gradenine.co.uk/glossary/kinetic-theory)

## Related

- [Revision notes for WJEC GCSE Physics (Wales)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
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