# A sealed metal canister contains a fixed mass of gas. The canister is placed in a freezer and left until the gas reaches a much lower temperature. Explain what happens to the average kinetic energy of the gas molecules and to the pressure of the gas inside the canister as it cools, in terms of the behaviour of the molecules.

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

## Mark scheme (5 marks)

1. The average kinetic energy of the gas molecules decreases (as temperature falls).
2. The molecules move more slowly (lower average speed).
3. The molecules collide with the walls of the canister less frequently.
4. Each collision with the wall exerts a smaller force (because molecules are moving more slowly / have less momentum).
5. Therefore the pressure of the gas decreases (because pressure is caused by the force of molecular collisions on the walls).

## Key terms

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

## 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-metal-canister-contains-a-e9beb0bd) · Published by Druglandscape Ltd.