A sealed balloon is filled with a fixed quantity of gas at room temperature. The balloon is then placed in a freezer. Explain, using kinetic theory, why the volume of the balloon decreases when it is placed in the freezer.

WJEC GCSE Physics (Wales) — 2.2 The kinetic theory and gas pressure · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

Lowering the temperature reduces the average kinetic energy of the gas molecules.
The molecules move more slowly and have lower speeds.
They collide with the balloon walls less frequently and with less force.
The pressure exerted by the gas on the walls falls, so the balloon shrinks until the internal pressure equals the external atmospheric pressure.

Examiner tips

  • Use the phrase "lowering the temperature" to link to kinetic theory. Show the chain: ↓T → ↓KE → ↓speed → ↓collision frequency/force → ↓pressure → shrinkage.
  • Mention that the balloon will stop shrinking when internal pressure equals external pressure.

Mark scheme (4 marks)

  1. Lowering the temperature decreases the average kinetic energy of the gas molecules
  2. The molecules move more slowly / with lower speeds
  3. The molecules collide with the walls of the balloon less frequently and/or with less force
  4. The pressure exerted by the gas on the balloon walls decreases, so the balloon shrinks until the internal pressure equals the external (atmospheric) pressure

Key terms in this question

kinetic theory · temperature · volume

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