# A weather balloon is filled with a fixed mass of gas at ground level. As the balloon rises to a high altitude, the surrounding atmospheric pressure decreases whilst the temperature of the gas remains approximately constant. Explain, in terms of gas molecules, why the volume of the balloon increases as it rises to a higher altitude.

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

> A weather balloon is released at ground level where the atmospheric pressure is high. As it ascends, the atmospheric pressure decreases. The temperature of the gas inside the balloon remains approximately constant throughout the ascent.

## Mark scheme (4 marks)

1. At constant temperature the molecules move at the same average speed / have the same average kinetic energy
2. As the volume increases, the molecules collide less frequently with the walls of the balloon
3. The molecules also collide with the walls over a greater area
4. This results in a lower pressure exerted by the gas on the balloon walls, matching the lower external atmospheric pressure / pressure decreases as volume increases (Boyle's Law / inverse relationship)

## Key terms

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

## 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-weather-balloon-is-filled-with-c6a3c0bf) · Published by Druglandscape Ltd.