A student pours a small amount of perfume onto a cotton wool ball and places it in the corner of a room. After a short time, the scent of the perfume can be detected on the opposite side of the room. Explain, using the particle model, why the perfume can be smelt across the room, and why the scent reaches the far side of the room more quickly on a warm day than on a cold day.

OCR GCSE Physics A: Gateway Science (J249) — P1.1 The particle model · Explain · 4 marks · View as Markdown

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

Perfume is a liquid at room temperature. When it evaporates, its particles enter the air as a gas.

Model answer (4 marks)

The perfume evaporates, so its molecules change from liquid to gas and enter the air.
The gas molecules move randomly in all directions, carrying the scent across the room.
On a warm day the molecules have greater kinetic energy and move faster.
The faster‑moving molecules spread across the room more quickly, so the scent reaches the far side sooner.

Examiner tips

  • Use the word "evaporates" to show state change. Mention "random motion" and "all directions" to hit the spread point. Explain kinetic energy increase with temperature. Link faster motion to quicker arrival.

Common mistakes

  • Saying the perfume simply "stays" in the air instead of evaporating. Forgetting to mention that motion is random. Mixing up temperature with pressure effects.

Mark scheme (4 marks)

  1. The perfume evaporates / changes from liquid to gas, so its particles enter the air
  2. Gas particles move / spread out randomly in all directions, carrying the scent across the room
  3. On a warmer day, the particles have greater kinetic energy / move faster
  4. Faster-moving particles spread across the room in less time, so the scent arrives more quickly

Key terms in this question

particle model

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