Explain why a liquid has a fixed volume but can change its shape to fill the bottom of any container it is placed in.

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)

Particles in a liquid are close together and in contact, so the volume is fixed and it cannot be compressed.
The intermolecular forces are strong enough to keep the particles at a similar distance, preventing expansion or compression.
Particles in a liquid can move, flow and slide past one another.
Because they can flow, the liquid takes the shape of its container and fills the bottom.

Examiner tips

Use the term ‘intermolecular forces’ and ‘particles in contact’ to earn full marks for the fixed volume part.
Show that particles can move/flow to justify the shape change.
Keep the answer concise – 4 points can be covered in 4 short sentences.

Common mistakes

Claiming a liquid has a fixed shape instead of a fixed volume.
Forgetting to mention the role of intermolecular forces.
Using vague terms like ‘particles are close’ without specifying contact or forces.

Mark scheme (4 marks)

Particles in a liquid are close together / touching / in contact with each other, so the volume is fixed / cannot be compressed
The intermolecular forces in a liquid are strong enough to hold particles at a similar distance from one another (preventing expansion/compression), giving a fixed volume
Particles in a liquid are able to move / flow / slide past each other
Because particles can move/flow past each other, the liquid takes the shape of its container (fills the bottom)

Key terms in this question

liquid · fixed volume