# Use the kinetic theory of matter to explain why solids have a fixed shape and cannot be compressed, whereas gases have no fixed shape and can be compressed easily.

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

## Mark scheme (4 marks)

1. In a solid, particles are held in fixed positions (by strong forces / in a regular arrangement / lattice)
2. This means solids have a fixed shape because particles cannot move from their positions / can only vibrate about fixed points
3. In a gas, particles are far apart / widely spaced with large gaps between them
4. Gas particles can be pushed closer together (into the gaps) so the gas can be compressed / has no fixed shape because particles move freely in all directions

## Key terms

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

## 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)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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