# A student puts a small amount of water into a syringe and then seals the nozzle. The student slowly pulls the plunger outwards, increasing the volume inside the syringe. The water turns into water vapour. Explain, using kinetic theory, why the water turns into water vapour when the volume is increased.

> WJEC A-Level Physics (Wales) — 3.3 Kinetic theory · Explain · 5 marks

## Mark scheme (5 marks)

1. The water molecules are in constant random motion
2. Increasing the volume increases the space available, so the pressure inside the syringe decreases
3. At lower pressure, the boiling point of water is reduced / water can boil at a lower temperature
4. The molecules in the liquid gain enough energy to overcome the forces of attraction between them
5. Molecules escape from the liquid and move freely as a gas / vapour

## Key terms

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

## 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-student-puts-a-small-amount-6330f32d) · Published by Druglandscape Ltd.