# Explain why the pressure exerted by a real gas is lower than that predicted by the ideal gas equation at high pressures.

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

## Mark scheme (4 marks)

1. The ideal gas model assumes no intermolecular forces between particles.
2. At high pressures, molecules are forced closer together, so intermolecular (attractive) forces become significant.
3. Attractive forces pull molecules toward the bulk of the gas, reducing the velocity/momentum with which they strike the container walls.
4. Therefore the measured pressure is less than the value calculated using the ideal gas equation (PV = nRT).

## Key terms

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

## Related

- [Revision notes for IB DP Chemistry Higher 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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