A weather balloon is filled with a fixed mass of gas at ground level. As the balloon rises to a high altitude, the surrounding atmospheric pressure decreases whilst the temperature of the gas remains approximately constant. Explain, in terms of gas molecules, why the volume of the balloon increases as it rises to a higher altitude.

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 weather balloon is released at ground level where the atmospheric pressure is high. As it ascends, the atmospheric pressure decreases. The temperature of the gas inside the balloon remains approximately constant throughout the ascent.

Model answer (4 marks)

At constant temperature the gas molecules have the same average kinetic energy and therefore move at the same average speed.

As the balloon rises the external pressure falls, so the gas expands. The expansion increases the volume, which means the molecules are spread over a larger space.

With a larger volume the molecules collide with the balloon walls less frequently, and they strike a greater area of the wall.

The reduced collision frequency and larger area give a lower pressure inside the balloon, which matches the lower atmospheric pressure at high altitude (Boyle’s law).

Examiner tips

  • Use the phrase ‘average kinetic energy’ to show understanding of constant temperature.
  • Explain the link between increased volume, fewer collisions and lower pressure.
  • Mention Boyle’s law to justify the inverse relationship.
  • Keep the answer concise and use the exact terminology from the mark scheme.

Common mistakes

  • Confusing temperature with kinetic energy – students say ‘speed’ only.
  • Forgetting to mention that the area of impact increases.
  • Failing to state that the internal pressure must equal the external pressure at equilibrium.

Mark scheme (4 marks)

  1. At constant temperature the molecules move at the same average speed / have the same average kinetic energy
  2. As the volume increases, the molecules collide less frequently with the walls of the balloon
  3. The molecules also collide with the walls over a greater area
  4. This results in a lower pressure exerted by the gas on the balloon walls, matching the lower external atmospheric pressure / pressure decreases as volume increases (Boyle's Law / inverse relationship)

Key terms in this question

pressure · volume

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