# Explain what happens to the molecules and the internal energy of a fixed mass of gas when it is compressed at constant temperature.

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

## Mark scheme (5 marks)

1. The molecules are closer together / the average distance between molecules decreases
2. The volume decreases so molecules hit the walls more frequently / there are more collisions with the walls per second
3. Temperature is constant so the average kinetic energy of the molecules does not change
4. The internal energy of a gas is the sum of the kinetic energy and potential energy of the molecules / internal energy depends on both kinetic and potential energy
5. The potential energy of the molecules changes (increases) as they are forced closer together, so the internal energy of the gas changes (increases)

## Key terms

- [internal energy](https://www.gradenine.co.uk/glossary/internal-energy)
- [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-what-happens-to-the-molecules-0c9f5528) · Published by Druglandscape Ltd.