# Explain why the volume of an ideal gas doubles when its pressure is halved at constant temperature.

> IB DP Chemistry Standard Level (2023 syllabus) — S1.5 Ideal gases · Explain · 4 marks

## Mark scheme (4 marks)

1. In an ideal gas, particles are in constant random motion and exert pressure through collisions with the container walls.
2. At constant temperature, the average kinetic energy (and speed) of the particles remains unchanged, so the force per collision stays the same.
3. Halving the pressure means the frequency of collisions per unit area of wall must halve.
4. Doubling the volume spreads the same number of particles over a greater space, so collisions with any given area of wall occur half as often, which halves the pressure — consistent with Boyle's law (P ∝ 1/V at constant T).

## Key terms

- [ideal gas](https://www.gradenine.co.uk/glossary/ideal-gas)
- [pressure](https://www.gradenine.co.uk/glossary/pressure)
- [constant temperature](https://www.gradenine.co.uk/glossary/constant-temperature)

## Related

- [Revision notes for IB DP Chemistry 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-why-the-volume-of-an-ac9048a1) · Published by Druglandscape Ltd.