# A fixed amount of gas is trapped inside a sealed bicycle pump with the nozzle blocked. A cyclist pushes the handle of the pump inward, compressing the gas into a smaller volume. The temperature of the gas does not change. Explain, using kinetic theory, why the pressure of the gas increases when its volume is decreased at constant temperature.

> WJEC A-Level Physics (Wales) — 4.2 Kinetic theory and gases (A-Level only) · Explain · 5 marks

> When a gas is compressed into a smaller volume at constant temperature, its pressure increases. This relationship can be explained using the kinetic theory of gases.

## Mark scheme (5 marks)

1. The gas is made up of moving particles (molecules/atoms) that collide with the walls of the container.
2. Pressure is caused by the force of these collisions on the walls.
3. When the volume decreases, the particles are closer together / confined to a smaller space.
4. The particles collide with the walls more frequently (more collisions per second).
5. Because temperature is unchanged, the speed/kinetic energy of the particles stays the same, so the increased collision frequency alone causes a greater force on the walls and therefore higher pressure.

## Key terms

- [kinetic theory](https://www.gradenine.co.uk/glossary/kinetic-theory)
- [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 WJEC A-Level Physics (Wales)](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/a-fixed-amount-of-gas-is-c91424cc) · Published by Druglandscape Ltd.