A small amount of perfume is sprayed at one end of a long room. After a few minutes, the scent can be detected at the other end of the room. Explain why the scent spreads through the room and why it would spread more slowly on a cold day than on a warm day.

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)

Perfume molecules are released into the air and therefore occupy a region of high concentration. They are in constant, random motion and move from the area of high concentration to the area of low concentration, spreading through the room by diffusion.

On a cold day the molecules have less kinetic energy and therefore move more slowly. Because the molecules move more slowly, it takes longer for them to travel the same distance, so the diffusion of the perfume is slower.

Examiner tips

  • Use the phrase ‘high concentration to low concentration’ to show understanding of diffusion direction. Explain that molecules are in constant random motion. Mention kinetic energy and its effect on speed. Link slower speed to slower diffusion.
  • common_mistakes
  • :
  • Forgetting to mention the high‑to‑low concentration gradient. Using vague terms like ‘mixing’ instead of ‘diffusion’. Neglecting to explain the role of kinetic energy or temperature.

Mark scheme (4 marks)

  1. Perfume molecules move/spread from an area of high concentration to an area of low concentration
  2. Perfume molecules are in constant/random motion
  3. On a cold day, perfume molecules have less (kinetic) energy / move more slowly
  4. Slower-moving molecules take longer to spread/diffuse across the room, so diffusion is slower

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