# Explain, using the kinetic molecular model, why the volume of a fixed mass of ideal gas increases when its temperature is raised at constant pressure.

> IB DP Physics Standard Level (2023 syllabus) — B.3 Gas laws · Explain · 4 marks

> A sealed balloon is placed in warm water. The balloon expands noticeably as the water temperature rises, while the pressure inside remains equal to atmospheric pressure throughout.

## Mark scheme (4 marks)

1. Raising the temperature increases the average kinetic energy of the gas molecules.
2. Faster-moving molecules strike the balloon walls with greater force / greater impulse per collision.
3. This tends to increase the pressure, so the balloon walls expand outward until the pressure returns to its original (atmospheric) value.
4. The larger volume means molecules travel further between collisions with the walls / collisions with the walls become less frequent, compensating for the greater force per collision and maintaining constant pressure.

## Key terms

- [kinetic molecular model](https://www.gradenine.co.uk/glossary/kinetic-molecular-model)
- [pressure](https://www.gradenine.co.uk/glossary/pressure)
- [constant pressure](https://www.gradenine.co.uk/glossary/constant-pressure)

## Related

- [Revision notes for IB DP Physics Standard Level (2023 syllabus)](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-using-the-kinetic-molecular-model-c4d15401) · Published by Druglandscape Ltd.