# Explain why a real gas such as ammonia deviates more significantly from ideal gas behaviour at high pressure and low temperature than at low pressure and high temperature.

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

## Mark scheme (4 marks)

1. An ideal gas assumes no intermolecular forces between particles; real gas molecules (e.g. ammonia) do experience intermolecular forces / attractions.
2. At low temperature, molecules move more slowly / have lower kinetic energy, so intermolecular attractions have a greater effect relative to kinetic energy, causing greater deviation.
3. An ideal gas assumes the volume of gas particles themselves is negligible compared to the container volume; real gas molecules occupy a finite / non-negligible volume.
4. At high pressure, molecules are forced closer together, so the actual volume occupied by the molecules becomes a significant fraction of the total volume, causing greater deviation from ideal behaviour.

## Key terms

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

## 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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