A sealed metal canister contains a fixed mass of gas. The canister is placed in boiling water, causing the temperature of the gas to rise from 27 °C to 127 °C. Explain why the pressure of the gas inside the canister increases.

Pearson Edexcel International GCSE Physics (4PH1) — 5.3 Ideal gas molecules · Explain · 4 marks · View as Markdown

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

A sealed metal canister contains a fixed mass of gas at 27 °C and a pressure of 100 kPa. The canister is placed in boiling water and the temperature of the gas rises to 127 °C. The volume of the canister does not change.

Model answer (4 marks)

The temperature rise makes the gas molecules move faster. Faster molecules collide with the walls more often and with greater force. Because pressure is the force per unit area on the walls, the increased frequency and force of collisions raise the pressure inside the canister, even though the volume is unchanged.

Examiner tips

  • Use the exact terms: ‘move faster’, ‘collide more frequently’, ‘collide harder’, ‘pressure is force per unit area’.
  • Show the logical chain: temperature → speed → collision frequency/force → pressure.
  • Keep the answer short – one sentence per point to hit all four marks.

Common mistakes

  • Saying the gas expands instead of noting the volume is constant.
  • Using vague phrases like ‘more energy’ without linking to collision frequency or force.
  • Omitting the definition of pressure as force per unit area.

Mark scheme (4 marks)

  1. The gas molecules move faster (at higher temperature)
  2. The molecules collide with the walls more frequently
  3. The molecules collide with the walls harder / with greater force
  4. Pressure is the force per unit area on the walls, so more frequent and harder collisions increase the pressure (at constant volume)

Key terms in this question

pressure

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