Explain, using particle theory, why a solid substance keeps a fixed shape but a liquid takes the shape of its container.

Edexcel GCSE Chemistry (1CH0) — 2.1 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 solid the particles are arranged in a regular, fixed lattice.
They can only vibrate about their fixed positions, so the overall shape cannot change.
In a liquid the particles are arranged randomly and are still close together.
The particles can move past one another, so the liquid can flow and take the shape of its container.

Examiner tips

Use the phrase ‘regular arrangement’ for solids and ‘random arrangement’ for liquids. Mention that solids vibrate only, liquids move past each other. Show the link between particle motion and macroscopic shape.

Common mistakes

Confusing ‘fixed shape’ with ‘fixed volume’. Saying particles in a liquid are ‘far apart’. Omitting the point that liquid particles can flow past each other.

Mark scheme (4 marks)

In a solid, particles are held in a regular arrangement
Solid particles can only vibrate about fixed positions, so the overall shape cannot change
In a liquid, particles are arranged randomly and close together
Liquid particles are able to move past each other, so the liquid can flow and fill the shape of the container

Key terms in this question

particle theory · solid · liquid