A sealed metal can contains a fixed amount of air. The can is placed in a freezer and left until its temperature drops significantly. Explain why the pressure of the air inside the can decreases as the temperature falls.

OCR GCSE Physics A: Gateway Science (J249) — P1.3 Pressure · Explain · 4 marks · View as Markdown

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

A food manufacturer stores sealed metal cans of product in industrial freezers. Engineers notice that the sides of the cans appear to be pushed slightly inward after freezing.

Model answer (4 marks)

A decrease in temperature reduces the kinetic energy of the gas particles, so they move more slowly.
The slower particles collide with the walls of the can less frequently.
Each collision is also less forceful because the particles have lower momentum.
The net result is a lower perpendicular force per unit area on the walls, i.e. a lower pressure inside the can.

Examiner tips

  • Use the word "decrease" or "reduce" to link temperature and pressure. Show the chain: lower temperature → lower kinetic energy → slower particles → fewer, less forceful collisions → lower pressure. Mention the pressure–force–area relationship explicitly.

Common mistakes

  • Confusing the number of collisions with the force of each collision. Forgetting to state that the pressure is the force per unit area. Using vague terms like "less energy" without linking to kinetic energy or collision frequency.

Mark scheme (4 marks)

  1. Decreasing temperature means the particles have less kinetic energy / move more slowly
  2. Particles collide with the walls of the can less frequently
  3. The collisions with the walls are also less forceful
  4. The perpendicular force per unit area on the walls is therefore reduced, so pressure decreases

Key terms in this question

pressure · temperature

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