Explain why gases are much more compressible than liquids.

IB DP Chemistry Higher Level (2023 syllabus) — S1.1 Introduction to the particulate nature 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, separated by large distances relative to their size. When pressure is applied the particles are forced closer together, reducing the space between them and therefore the volume decreases. In a liquid the particles are already very close together, essentially touching with negligible free space. Consequently there is little or no space left to reduce between the particles, so the volume cannot decrease significantly under pressure.

Examiner tips

Use the term ‘far apart’ for gases and ‘touching’ for liquids; mention ‘free space’ or ‘inter‑particle distance’.
Show the causal link: pressure → reduced inter‑particle distance → volume change.
Keep the answer concise – 4 marks, so one sentence per point is sufficient.

Common mistakes

Confusing the behaviour of solids with liquids; stating that liquids are compressible like gases.
Using vague phrases such as ‘particles are close’ for both gases and liquids, or omitting the role of pressure.

Mark scheme (4 marks)

In a gas, particles are far apart / separated by large distances compared to the size of the particles themselves.
Applying pressure forces the gas particles closer together / reduces the space between particles, so the volume decreases.
In a liquid, particles are already very close together / touching / in contact, with negligible space between them.
Therefore, there is little/no space available to reduce between liquid particles, so the volume cannot decrease significantly under pressure.

Key terms in this question

gas · liquid

More Introduction to the particulate nature of matter questions

▶ Try answering this question with AI marking (free) →