# Explain, using the kinetic molecular model, why the pressure of an ideal gas remains constant when its volume is decreased and its temperature is simultaneously reduced.

> IB DP Physics Standard Level (2023 syllabus) — B.3 Gas laws · Explain · 4 marks

> A fixed mass of ideal gas is compressed slowly in a thermally insulated container. A student then carefully reduces the thermal insulation and allows heat to escape, so that the temperature of the gas falls back to its original value. The student observes that the pressure returns to exactly its original value as well.

## Mark scheme (4 marks)

1. Decreasing the volume alone would increase the frequency of molecular collisions with the walls, tending to increase pressure.
2. Decreasing the temperature alone would reduce the average kinetic energy (and hence average speed) of the molecules, tending to decrease pressure.
3. The two effects exactly cancel: the increase in collision frequency due to reduced volume is offset by the decrease in force per collision due to reduced temperature.
4. This is consistent with the ideal gas law: if the temperature and volume are both reduced by the same factor, the pressure P = nRT/V remains unchanged.

## Key terms

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

## Related

- [Revision notes for IB DP Physics Standard Level (2023 syllabus)](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/explain-using-the-kinetic-molecular-model-5d777546) · Published by Druglandscape Ltd.