A metal tin contains air at room temperature. The tin is placed in a freezer and left until the air inside cools to a much lower temperature. Explain, using kinetic theory, why the pressure of the air inside the tin decreases when it is cooled.

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)

Air is made up of molecules that are in constant random motion.
Pressure is produced by the collisions of these molecules with the walls of the tin.
When the tin is cooled the temperature of the air falls, so the average kinetic energy of the molecules decreases.
The molecules then move more slowly and each collision with the wall exerts a smaller force.
Because the molecules are moving more slowly there are also fewer collisions per second, so the overall pressure inside the tin decreases.

Examiner tips

  • Use the phrase ‘average kinetic energy’ to link temperature and speed. Show the chain: lower T → lower KE → slower molecules → less force per collision and fewer collisions → lower pressure. Include the word ‘molecules’ or ‘particles’ to meet the first point.
  • Use the exact wording from the mark scheme where possible – e.g. ‘pressure is caused by particles colliding with the walls’ – to secure the full 5 marks.

Common mistakes

  • Forgetting to mention that pressure comes from collisions with the walls. Using ‘atoms’ instead of ‘molecules’ for air. Failing to explain that both the force per collision and the collision frequency decrease.

Mark scheme (5 marks)

  1. Air is made up of particles (molecules/atoms) that are in constant random motion
  2. Pressure is caused by particles colliding with the walls of the tin
  3. When the temperature decreases, the particles move more slowly / have less kinetic energy
  4. Slower particles hit the walls with less force
  5. Fewer collisions per second with the walls (at lower temperature), so overall pressure is lower

Key terms in this question

kinetic theory · pressure · temperature

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