# A gas is trapped inside a rigid steel cylinder. The cylinder is placed in a freezer and left until the gas reaches a much lower temperature. Explain, in terms of the motion and behaviour of gas molecules, why the pressure of the gas decreases as the temperature falls.

> WJEC A-Level Physics (Wales) — 4.2 Kinetic theory and gases (A-Level only) · Explain · 5 marks

## Mark scheme (5 marks)

1. Gas molecules are in constant random motion
2. Pressure is caused by molecules colliding with the walls of the container
3. At lower temperature, molecules move more slowly / have less kinetic energy
4. Molecules collide with the walls less frequently OR with less force per collision
5. Therefore the overall force on the walls is less, so pressure decreases

## Key terms

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

## Related

- [Revision notes for WJEC A-Level 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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Source: [GradeNine](https://www.gradenine.co.uk/q/a-gas-is-trapped-inside-a-77bcba37) · Published by Druglandscape Ltd.