# Explain, using the kinetic molecular model, why a fixed mass of ideal gas at constant temperature obeys Boyle's law.

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

> Boyle's law states that for a fixed mass of gas at constant temperature, pressure is inversely proportional to volume.

## Mark scheme (4 marks)

1. Temperature is constant, so the average kinetic energy (and therefore average speed) of the molecules is unchanged.
2. When volume decreases, the molecules are confined to a smaller space, so they travel shorter distances between collisions with the walls.
3. The increased frequency of collisions per unit area of the walls means the force per unit area (pressure) increases.
4. Since the increase in pressure is proportional to the decrease in volume (with no change in molecular speed), pressure and volume are inversely proportional, consistent with Boyle's law.

## Key terms

- [kinetic molecular model](https://www.gradenine.co.uk/glossary/kinetic-molecular-model)
- [Boyle's law](https://www.gradenine.co.uk/glossary/boyle-s-law)

## 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-ed43f494) · Published by Druglandscape Ltd.