Explain why the volume of a fixed amount of an ideal gas increases when its temperature is raised at constant pressure.

IB DP Chemistry Standard Level (2023 syllabus) — S1.5 Ideal gases · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

Raising the temperature increases the average kinetic energy of the gas particles. The faster particles collide with the walls more frequently and/or with greater force, which would increase the pressure. Because the pressure is held constant, the gas expands – its volume increases – so that the rate of collisions per unit area remains the same. This is Charles’s law: at constant pressure the volume is directly proportional to the absolute temperature.

Examiner tips

Use the word ‘average kinetic energy’ or ‘speed’ to show understanding of temperature effect.
Explain the link between collision frequency/force and pressure, then state the volume change required to keep pressure constant.
Mention Charles’s law to show the proportional relationship.
Keep the answer concise – 4 marks can be earned with 3–4 short sentences.

Common mistakes

Saying the volume increases because pressure increases – the pressure is actually held constant.
Using ‘temperature rise causes pressure rise’ without explaining the need for volume change.
Failing to mention Charles’s law or the proportionality to Kelvin temperature.

Mark scheme (4 marks)

Raising temperature increases the average kinetic energy (or speed) of the gas particles.
Faster-moving particles collide with the container walls more frequently and/or with greater force, tending to increase pressure.
Because pressure is held constant, the volume must increase to restore (keep) the same rate/force of collisions per unit area.
This is consistent with Charles's law: volume is directly proportional to absolute (kelvin) temperature at constant pressure.

Key terms in this question

ideal gas · pressure