# A fixed mass of gas is trapped inside a syringe at constant temperature. The piston of the syringe is slowly pushed inward, reducing the volume of the gas. Explain why the pressure of the gas increases as the volume decreases.

> Pearson Edexcel International GCSE Physics (4PH1) — 5.3 Ideal gas molecules · Explain · 4 marks

> A fixed mass of gas is trapped inside a syringe at constant temperature. The piston is slowly pushed inward so that the volume of the gas is reduced.

## Mark scheme (4 marks)

1. The gas molecules collide with the walls of the syringe (and piston) more frequently
2. Because the same number of molecules occupy a smaller volume / the molecules have less space to travel before hitting a wall
3. The collisions also occur over a smaller (surface) area
4. Greater frequency of collisions (and smaller area) means greater force per unit area, so pressure increases

## Key terms

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

## Related

- [Revision notes for Pearson Edexcel International GCSE Physics (4PH1)](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-mass-of-gas-is-d42427ae) · Published by Druglandscape Ltd.