# A fixed quantity of gas is trapped in a container at constant temperature. Explain, using kinetic theory, why the pressure of a gas increases when its volume is decreased.

> WJEC GCSE Physics (Wales) — 2.2 The kinetic theory and gas pressure · Explain · 4 marks

> When a gas is compressed into a smaller volume at constant temperature, the pressure of the gas increases. This is known as the pressure-volume relationship.

## Mark scheme (4 marks)

1. Gas particles collide with the walls of the container, producing pressure
2. When volume decreases, the particles are closer together / in a smaller space
3. The frequency of collisions with the walls increases
4. More frequent collisions mean a greater force per unit area, so pressure increases

## Key terms

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

---
Source: [GradeNine](https://www.gradenine.co.uk/q/a-fixed-quantity-of-gas-is-4d31a815) · Published by Druglandscape Ltd.