Explain why a gas can be compressed into a much smaller volume, but a solid cannot be compressed.

Pearson Edexcel International GCSE Chemistry (4CH1) — 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. These spaces can be reduced, so the particles can be pushed closer together and the gas can be compressed.
In a solid the particles are closely packed, touching each other and held in fixed positions. There are no significant spaces between the particles, so there is nothing to reduce and the solid cannot be compressed.

Examiner tips

Use the word ‘spaces’ or ‘gaps’ to show understanding of particle separation
Explain that compression reduces the spaces, not the particle size
Mention that solids have fixed positions, not just close packing

Common mistakes

Confusing the inability to compress solids with the inability to expand them
Saying solids are ‘rigid’ without linking to particle positions
Using vague terms like ‘tight’ instead of ‘closely packed’

Mark scheme (4 marks)

In a gas, particles are relatively spread out / far apart (with large spaces between them)
These spaces can be reduced / particles can be pushed closer together, so the gas can be compressed
In a solid, particles are closely packed / touching / held in fixed positions with no significant spaces between them
There are no spaces to reduce in a solid, so it cannot be compressed

Key terms in this question

gas · solid · compressed