Explain why a gas can be compressed much more easily than a liquid, using ideas about particles and the forces between them.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
In a gas the particles are far apart, so there are large spaces between them. The intermolecular forces are negligible, so the particles can move freely. When a gas is compressed the large spaces are reduced, allowing the particles to be pushed closer together with little resistance. In a liquid the particles are already close together and the intermolecular forces are strong. Because the particles are touching, any attempt to reduce the volume is resisted by these forces, making compression very difficult.
Examiner tips
- Use the key terms ‘far apart’, ‘negligible forces’, ‘large spaces’, and ‘strong forces’ to show understanding.
- Show the logical sequence: distance → force → compressibility.
- Mention that compression reduces volume by reducing the spaces between particles.
- Keep the answer concise – 4 marks only.
Common mistakes
- Confusing the behaviour of solids with liquids; forgetting that liquids are not easily compressible.
- Using vague language such as ‘particles are close’ without linking to forces.
- Failing to mention that gas particles are far apart and forces are weak.
Mark scheme (4 marks)
- In a gas, particles are far apart / have large distances between them (whereas in a liquid particles are close together).
- Intermolecular / interparticle forces in a gas are negligible / very weak (compared to those in a liquid).
- Compression reduces the volume / distance between particles; in a gas the large spaces allow the particles to be pushed closer together.
- In a liquid, particles are already close together / touching, so the strong intermolecular forces resist further reduction in volume, making compression very difficult.
Key terms in this question
Related
- All IB DP Chemistry Standard Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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