A student places a crystal of purple potassium manganate(VII) at the bottom of a beaker of cold water. After ten minutes, a faint purple colour is visible throughout the water. The student then repeats the experiment using hot water. Explain why the purple colour spreads through the hot water more quickly than through the cold water.

Cambridge International IGCSE Chemistry (0620) — 1.2 Diffusion · 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 movement of particles from an area of higher concentration to an area of lower concentration.
In hot water the water molecules and the potassium manganate(VII) ions have more kinetic energy.
Particles in hot water therefore move faster.
Consequently the potassium manganate(VII) particles spread or mix throughout the hot water more quickly than in cold water.

Examiner tips

  • Use the term ‘diffusion’ first, then explain the role of kinetic energy and particle speed.
  • Show the logical sequence: higher temperature → more kinetic energy → faster particles → faster diffusion.

Common mistakes

  • Confusing convection with diffusion; only diffusion is required.
  • Using vague phrases like ‘more mixing’ without linking to kinetic energy or particle speed.

Mark scheme (4 marks)

  1. Diffusion is the movement of particles from a region of higher concentration to a region of lower concentration
  2. In hot water the water (and potassium manganate(VII)) particles have more kinetic energy
  3. Particles in hot water move faster
  4. Therefore the potassium manganate(VII) particles spread / mix throughout the water more quickly

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