Explain why a gas can be compressed into a much smaller volume but a liquid cannot be compressed in the same way.

Edexcel GCSE Chemistry (1CH0) — 1.1 States of matter · Explain · 4 marks · View as Markdown

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, leaving large spaces between them. When a gas is compressed, the particles are pushed closer together, reducing the spaces between them. In a liquid the particles are already close together, touching or almost touching, so there is very little space between them. Consequently there are no spaces to reduce and the particles cannot be pushed any closer together, meaning the liquid cannot be compressed.

Examiner tips

Use the word ‘far apart’ for gases and ‘close together’ for liquids. Mention ‘spaces between particles’ and ‘reduced when compressed’. Show the contrast: gas can be compressed, liquid cannot. Keep answer concise to fit 4 marks.

Common mistakes

Confusing the behaviour of solids with liquids. Forgetting to mention the lack of space in liquids. Using vague terms like ‘dense’ instead of ‘close together’.

Mark scheme (4 marks)

In a gas, the particles are far apart / there are large spaces between particles
So when a gas is compressed, the particles are pushed closer together / the spaces between particles are reduced
In a liquid, the particles are already close together / touching / very little space between particles
So there are no spaces to reduce / the particles cannot be pushed any closer together, meaning the liquid cannot be compressed

Key terms in this question

compress · gas · liquid · volume