# A weather balloon is filled with a fixed mass of gas at ground level. As the balloon rises to a higher altitude, the surrounding air pressure decreases and the balloon expands. The temperature of the gas remains constant throughout. Explain why the pressure of the gas inside the balloon decreases as it expands.

> AQA GCSE Physics (8463) — 4.3.3 Particle model and pressure (Phys only) · Explain · 4 marks

## Mark scheme (4 marks)

1. The gas molecules are in constant random motion and collide with the walls of the balloon
2. The pressure is caused by the force exerted by these collisions on the walls (per unit area)
3. When the volume increases, the molecules are more spread out / the number of collisions per unit area of the wall decreases
4. Fewer collisions per unit area means less force per unit area, so the pressure is lower

## Key terms

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

## Related

- [Revision notes for AQA GCSE Physics (8463)](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-weather-balloon-is-filled-with-81374fac) · Published by Druglandscape Ltd.