A scientist seals a small piece of solid iodine inside a glass flask containing air at room temperature. After several hours, the entire flask appears filled with a purple-brown colour. Explain why the purple-brown colour spreads to fill the entire flask.

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)

Iodine sublimes, so the solid turns directly into a gas.
The iodine molecules are in constant random motion.
They diffuse, moving from the high‑concentration region near the solid to the lower‑concentration areas.
Because gas molecules are far apart, the iodine vapour can spread freely and fill the whole flask.

Examiner tips

  • Use the word ‘sublime’ to show understanding of phase change.
  • Explain that diffusion is random motion from high to low concentration.
  • Mention the large spaces between gas molecules that allow the spread.
  • Keep the answer concise – 4 points only.

Common mistakes

  • Confusing sublimation with melting or evaporation.
  • Forgetting to state that diffusion is from high to low concentration.
  • Using vague terms like ‘mixing’ without explaining the mechanism.

Mark scheme (4 marks)

  1. Iodine sublimes / turns directly from solid to vapour (gas), producing iodine molecules in the gas phase
  2. Iodine molecules (particles) move randomly / are in constant random motion
  3. Iodine molecules spread out / mix throughout the flask by diffusion, moving from a region of higher concentration to lower concentration
  4. Gas molecules have large spaces between them / are far apart, allowing iodine molecules to move freely and fill the available space

Related

More Diffusion questions

▶ Try answering this question with AI marking (free) →