# A fixed quantity of gas is trapped inside a syringe. The plunger is pushed in slowly so that the volume of the gas decreases, while the temperature of the gas stays the same. Explain, using kinetic theory, why the pressure of the gas increases as the volume decreases.

> WJEC GCSE Physics (Wales) — 2.2 The kinetic theory and gas pressure · Explain · 4 marks

## Mark scheme (4 marks)

1. The gas particles collide with the walls of the syringe, and these collisions cause the pressure of the gas.
2. As volume decreases, the particles are closer together / confined to a smaller space.
3. So the particles collide with the walls more frequently / more often.
4. More frequent collisions means a greater force per unit area on the walls, so the pressure is higher.

## Key terms

- [pressure](https://www.gradenine.co.uk/glossary/pressure)
- [kinetic theory](https://www.gradenine.co.uk/glossary/kinetic-theory)
- [volume](https://www.gradenine.co.uk/glossary/volume)
- [temperature](https://www.gradenine.co.uk/glossary/temperature)

## Related

- [Revision notes for WJEC GCSE 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-quantity-of-gas-is-be1c0422) · Published by Druglandscape Ltd.