# A sealed syringe contains a fixed mass of gas at room temperature. The plunger is pushed inward, reducing the volume of the gas. The temperature of the gas stays constant throughout. Explain, in terms of the molecules of the gas, why the pressure of the gas increases when its volume is decreased.

> WJEC A-Level Physics (Wales) — 4.1 Thermal physics · Explain · 5 marks

## Mark scheme (5 marks)

1. The gas molecules are moving (randomly) and collide with the walls of the syringe
2. Pressure is caused by these collisions (with the walls)
3. When volume decreases, the same number of molecules are in a smaller space
4. Molecules collide with the walls more frequently / more often
5. More frequent collisions means a greater force on the walls, so pressure is higher

## Key terms

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

## 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-sealed-syringe-contains-a-fixed-7727563a) · Published by Druglandscape Ltd.