A student places a drop of ink into a glass of still water at room temperature. Over several minutes, the ink spreads throughout the water without being stirred. Explain, using kinetic theory, why the ink spreads through the water.

Eduqas A-Level Physics — 3.3 Kinetic theory · Explain · 5 marks · View as Markdown

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

Model answer (5 marks)

Water and ink particles are in continuous random motion.
Ink particles collide with water particles and vice‑versa.
These collisions give the ink particles a range of unpredictable directions.
Over time the ink particles spread throughout the water – this is diffusion.
No external energy is required because the particles already possess kinetic energy.

Examiner tips

  • Use the word ‘diffusion’ to show understanding of the process.
  • Show the sequence: motion → collisions → random directions → spread.
  • Mention that no stirring is needed because kinetic energy is inherent.
  • Keep the answer concise – 5 marks can be earned with 5 clear points.

Common mistakes

  • Confusing convection with diffusion; only diffusion occurs in still water.
  • Omitting the word ‘diffusion’ or not linking collisions to spread.
  • Using vague terms like ‘mixing’ instead of ‘diffusion’.

Mark scheme (5 marks)

  1. Water (and ink) particles are in continuous random motion
  2. The ink particles collide with water particles (and vice versa)
  3. These collisions cause the ink particles to move in different/unpredictable directions
  4. Over time, ink particles spread throughout the water (this process is called diffusion)
  5. No energy input (stirring) is needed because the particles already have kinetic energy / are already moving

Key terms in this question

kinetic theory

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