# A sealed plastic syringe contains a fixed mass of air. The open end of the syringe is blocked and the plunger is pushed inward so that the volume of the air decreases. The temperature of the air stays constant throughout. Explain what happens to the pressure of the air inside the syringe as the volume decreases, in terms of the molecules of the gas.

> Eduqas A-Level Physics — 4.1 Thermal physics · Explain · 5 marks

## Mark scheme (5 marks)

1. The gas molecules move randomly / are in continuous random motion
2. Pressure is caused by molecules colliding with / hitting the walls of the syringe
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 mean the force on the walls increases, so pressure increases

## 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 Eduqas A-Level Physics](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-plastic-syringe-contains-a-64337cf4) · Published by Druglandscape Ltd.