Explain why a fixed mass of gas at constant temperature has a lower pressure when its volume is increased.

AQA A-Level Physics (7408) — 3.6.2 Thermal Physics · Explain · 5 marks · View as Markdown

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

Model answer (5 marks)

A fixed mass of gas contains a fixed number of molecules that move randomly. Pressure results from the momentum transferred when these molecules collide with the walls of the container. If the volume is increased, the same number of molecules occupy a larger space, so the average distance between successive collisions with the walls increases. Because the temperature is held constant, the average kinetic energy and speed of the molecules remain unchanged. Consequently, collisions with the walls become less frequent, reducing the pressure.

Examiner tips

Use the definition of pressure as momentum transfer per unit area; link volume change to collision frequency; mention constant temperature keeps kinetic energy constant.
Show the logical chain: more volume → more space between molecules → fewer wall collisions → lower pressure.
Use clear, concise language and the exact terminology (e.g., "molecules", "collide", "pressure").

Common mistakes

Confusing volume increase with temperature change; forgetting to state that kinetic energy remains constant.
Failing to explain that fewer collisions per unit time cause the pressure drop.
Using vague terms like "less force" instead of "fewer collisions per unit area”.

Mark scheme (5 marks)

Gas consists of particles (molecules/atoms) moving randomly
Pressure is caused by particles colliding with the walls of the container
When volume increases, particles are more spread out / the space between particles is greater
Temperature is constant so the average speed / kinetic energy of particles is unchanged
Particles collide with the walls less frequently, so pressure decreases

Key terms in this question

pressure · volume · temperature