# Explain why increasing the temperature of a fixed mass of gas at constant pressure causes its volume to increase.

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

> A sealed balloon is placed in a warm room. After a few hours, the balloon has noticeably expanded.

## Mark scheme (5 marks)

1. Increasing temperature increases the kinetic energy of the gas molecules
2. Faster-moving molecules hit the walls of the balloon more frequently and/or with greater force
3. This increases the pressure that the gas exerts on the walls
4. Because pressure must remain constant (equal to atmospheric pressure), the balloon expands
5. A larger volume means molecules travel further between collisions with the walls, reducing collision frequency back to the original rate, so pressure is restored to its original value

## 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-increasing-the-temperature-of-831cd592) · Published by Druglandscape Ltd.