# Explain, using the kinetic model of an ideal gas, why the pressure exerted by a fixed mass of ideal gas decreases when its volume is increased at constant temperature.

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

## Mark scheme (4 marks)

1. Gas molecules collide with the walls of the container, and the rate of these collisions produces the pressure.
2. At constant temperature, the average kinetic energy (and hence average speed) of the molecules remains unchanged.
3. When the volume increases, molecules must travel a greater distance between successive collisions with the walls.
4. The frequency of collisions per unit area of the wall therefore decreases, so the pressure decreases.

## Key terms

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

## Related

- [Revision notes for IB DP Physics Higher 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-model-of-48febd2b) · Published by Druglandscape Ltd.