A student places a drop of ink into a beaker of still water at room temperature. Over the next few minutes, the ink slowly spreads throughout the water until the colour is uniform. Explain why the ink spreads through the water, and why the process happens more quickly if the water is heated.

WJEC A-Level Physics (Wales) — 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 molecules are in continuous random motion.
Ink particles are carried by collisions with the water molecules, which push them in random directions (Brownian motion / diffusion).
Over time this results in the ink becoming evenly spread throughout the water.
When the water is heated the molecules have more kinetic energy and move faster.
Faster‑moving molecules collide with the ink particles more frequently and with greater force, so the ink spreads more quickly.

Examiner tips

  • Use the term ‘diffusion’ or ‘Brownian motion’ to show understanding of the mechanism.
  • Explain the role of kinetic energy and collision frequency when the water is heated.
  • Keep the answer concise and use the exact wording from the mark scheme.

Mark scheme (5 marks)

  1. Water molecules are in continuous random motion
  2. Water molecules collide with ink particles, pushing them in random directions (Brownian motion / diffusion)
  3. Over time this results in the ink becoming evenly spread / uniform throughout the water
  4. When the water is heated, the water molecules have more kinetic energy and move faster
  5. Faster-moving water molecules collide with ink particles more frequently / with greater force, so the ink spreads more quickly

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