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

Edexcel GCSE Chemistry (1CH0) — 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 widely spaced, so there is a large amount of free volume for them to move through.
In a liquid the particles are close together, almost touching, leaving very little free space.
Gas particles move randomly at high speeds, so they cover large distances between collisions.
In a liquid the particles collide frequently and cannot travel far before colliding, so diffusion is slower.

Examiner tips

Use the word ‘diffuse’ and explain the role of particle spacing and speed.
Show the contrast between ‘freely moving’ gas particles and ‘frequently colliding’ liquid particles.
Mention that faster movement and more space give faster diffusion.

Common mistakes

Confusing diffusion with conduction or convection.
Failing to mention both spacing and speed.
Using vague terms like ‘more particles’ instead of ‘more space’ or ‘higher speed’.

Mark scheme (4 marks)

In a gas, particles are far apart / have large spaces between them
In a liquid, particles are close together / touching / very little space between them
Gas particles move randomly and at high speed / move faster than liquid particles
In a liquid, particles collide more frequently / cannot travel as far before colliding, so mixing is slower

Key terms in this question

gaseous state · liquid state