Explain why gases can be compressed much more easily than liquids.

AQA A-Level Physics (7408) — 3.6.2 Thermal Physics · Explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

A bicycle pump contains air. When the nozzle is blocked and the handle is pushed in, the air is compressed. A sealed syringe filled with water cannot be compressed in the same way.

Model answer (5 marks)

In a gas the particles are widely spaced, leaving large gaps between them. When pressure is applied the gaps are reduced, so the volume decreases.
In a liquid the particles are almost touching, with almost no free space. Applying pressure cannot push the particles any closer, so the volume remains essentially unchanged.
Thus a gas can be compressed much more easily than a liquid.

Examiner tips

Use the key terms ‘particles are far apart’ and ‘almost touching’
Show the link between space between particles and compressibility
Explain the volume change for gas vs liquid

Common mistakes

Confusing the behaviour of solids with liquids
Failing to mention the lack of free space in liquids
Using vague language such as ‘more dense’ instead of ‘closer particles’

Mark scheme (5 marks)

In a gas, the particles are far apart (with large spaces between them)
In a liquid, the particles are close together / touching / almost touching
Compressing a gas reduces the space / volume between the particles
In a liquid there is (almost) no space between particles so they cannot be pushed closer together
Therefore a gas can have its volume reduced but a liquid's volume stays (almost) constant when pressure is applied

Key terms in this question

gas · liquid · compressed