# Explain, using the kinetic molecular model, why the pressure exerted by a fixed mass of ideal gas decreases when some of the gas is allowed to escape from a rigid container at constant temperature.

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

## Mark scheme (4 marks)

1. Pressure arises from collisions of gas molecules with the container walls
2. When gas escapes, the number of molecules inside the container decreases
3. With fewer molecules, there are fewer collisions per unit time per unit area with the walls
4. Since temperature is constant, average molecular speed / kinetic energy is unchanged, so the reduction in collision frequency alone accounts for the lower pressure

## Key terms

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

## 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-d690a7b7) · Published by Druglandscape Ltd.