Describe what is meant by diffusion and explain why diffusion occurs more quickly in gases than in liquids.

Cambridge International IGCSE Chemistry (0620) — 1.2 Diffusion · Describe and Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, caused by the random motion of the particles.

In a gas the particles are further apart and move faster (higher kinetic energy) than in a liquid. Because collisions are less frequent, a gas particle can travel a longer distance before being impeded, so diffusion occurs more quickly in gases than in liquids.

Examiner tips

  • Use the definition of diffusion first, then explain the two key differences – particle spacing and speed – that give gases a higher rate.
  • Show the link between random motion, kinetic energy and collision frequency.
  • Keep the answer concise and use the exact wording from the mark scheme.

Mark scheme (4 marks)

  1. Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration
  2. Diffusion involves random movement of particles
  3. In gases, particles are further apart / have greater spaces between them than in liquids
  4. Gas particles move faster / have higher kinetic energy than liquid particles (at the same temperature), so collisions are less frequent, allowing particles to travel further before being impeded

Key terms in this question

diffusion

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