# Explain why a fixed mass of gas at constant temperature has a lower pressure when its volume is increased.

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

## Mark scheme (5 marks)

1. Gas consists of particles (molecules/atoms) moving randomly
2. Pressure is caused by particles colliding with the walls of the container
3. When volume increases, particles are more spread out / the space between particles is greater
4. Temperature is constant so the average speed / kinetic energy of particles is unchanged
5. Particles collide with the walls less frequently, so pressure decreases

## 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-fixed-mass-of-824b938b) · Published by Druglandscape Ltd.