# A weather balloon is filled with a fixed mass of gas at ground level, where the temperature is 17 °C and the pressure is high. As the balloon rises into the upper atmosphere, the surrounding pressure decreases and the temperature drops to −33 °C. Explain what happens to the volume of the gas inside the balloon as it rises, considering separately the effect of decreasing pressure and the effect of decreasing temperature.

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

> Weather balloons are used to carry scientific instruments high into the atmosphere. The balloon is only partially inflated at ground level before release.

## Mark scheme (5 marks)

1. As pressure decreases (at constant temperature), the gas molecules hit the walls of the balloon less frequently per unit area / the walls are pushed outward more / volume increases
2. Pressure and volume are inversely proportional (for a fixed mass of gas at constant temperature), so lower pressure means larger volume
3. As temperature decreases, the gas molecules move more slowly / have less kinetic energy
4. Slower-moving molecules exert less force/pressure on the walls, so the volume decreases (opposing the pressure effect)
5. Overall the volume increases because the effect of the large pressure decrease dominates over the effect of the temperature decrease / the balloon expands as it rises

## Key terms

- [pressure](https://www.gradenine.co.uk/glossary/pressure)
- [volume](https://www.gradenine.co.uk/glossary/volume)
- [temperature](https://www.gradenine.co.uk/glossary/temperature)
- [fixed mass of gas](https://www.gradenine.co.uk/glossary/fixed-mass-of-gas)

## 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-weather-balloon-is-filled-with-a9c63a6b) · Published by Druglandscape Ltd.