Explain why a substance in the gaseous state diffuses much more quickly than the same substance in the liquid state.

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).

Diffusion is the movement of particles from a region of higher concentration to a region of lower concentration. It occurs in both liquids and gases, but at very different rates.

Model answer (4 marks)

Gas particles move randomly in all directions at very high speeds, much faster than liquid particles.
Gas particles are widely spaced, so there are fewer collisions and less obstruction between them.
Liquid particles are closely packed, so collisions are frequent and slow diffusion.
Stronger intermolecular forces in a liquid restrict the movement of its particles, further reducing the rate of diffusion.

Examiner tips

Use the word ‘diffusion’ and explain both speed and spacing. Mention both kinetic energy and intermolecular forces. Keep the answer concise – 4 points can be covered in 4 short sentences.

Common mistakes

Confusing diffusion with evaporation or osmosis. Failing to mention the role of intermolecular forces. Using vague terms like ‘fast’ without linking to kinetic energy or particle spacing.

Mark scheme (4 marks)

Gas particles move randomly / in all directions at high speed (much faster than liquid particles)
Gas particles are widely spaced / far apart, so there are fewer collisions / less obstruction between particles
Liquid particles are closely packed / close together, so collisions between particles are frequent, slowing diffusion
Intermolecular forces in a liquid are stronger than in a gas, restricting the movement of liquid particles

Key terms in this question

gas · liquid