# Explain why a gas exerts a pressure on the walls of its container, and why this pressure increases when the same mass of gas is compressed into a smaller volume at constant temperature.

> AQA A-Level Physics (7408) — 3.6.2 Thermal Physics · Explain · 5 marks

## Mark scheme (5 marks)

1. Gas molecules are in continuous (random) motion
2. Molecules collide with the walls of the container
3. Each collision exerts a force on the wall, and pressure arises from these forces (over the area of the wall)
4. When volume decreases (at constant temperature), molecules are closer together / have less space to move
5. Molecules collide with the walls more frequently, so pressure increases

## Key terms

- [pressure](https://www.gradenine.co.uk/glossary/pressure)
- [volume](https://www.gradenine.co.uk/glossary/volume)
- [temperature](https://www.gradenine.co.uk/glossary/temperature)

## Related

- [Revision notes for AQA A-Level Physics (7408)](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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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-why-a-gas-exerts-a-843326e4) · Published by Druglandscape Ltd.