Explain, using the kinetic molecular model, why the volume of a fixed mass of ideal gas increases when its temperature is raised at constant pressure.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A sealed balloon is placed in warm water. The balloon expands noticeably as the water temperature rises, while the pressure inside remains equal to atmospheric pressure throughout.
Model answer (4 marks)
Raising the temperature increases the average kinetic energy of the gas molecules.
The faster‑moving molecules collide with the balloon walls with greater force (greater impulse per collision).
This tends to increase the pressure, so the balloon expands until the pressure again equals the atmospheric pressure.
The larger volume means the molecules travel further between collisions with the walls, so collisions become less frequent, which compensates for the greater force per collision and keeps the pressure constant.
The faster‑moving molecules collide with the balloon walls with greater force (greater impulse per collision).
This tends to increase the pressure, so the balloon expands until the pressure again equals the atmospheric pressure.
The larger volume means the molecules travel further between collisions with the walls, so collisions become less frequent, which compensates for the greater force per collision and keeps the pressure constant.
Examiner tips
- Use the kinetic‑molecular model terms (average kinetic energy, force/impulse, collisions).
- Show the cause–effect chain: ↑T → ↑KE → ↑force per collision → ↑pressure → expansion → constant P.
- Mention that the expansion reduces collision frequency to maintain P.
Common mistakes
- Failing to mention the role of collision frequency or force per collision.
- Using vague terms like ‘more energy’ without linking to kinetic energy or pressure.
- Not stating that the pressure remains atmospheric after expansion.
Mark scheme (4 marks)
- Raising the temperature increases the average kinetic energy of the gas molecules.
- Faster-moving molecules strike the balloon walls with greater force / greater impulse per collision.
- This tends to increase the pressure, so the balloon walls expand outward until the pressure returns to its original (atmospheric) value.
- The larger volume means molecules travel further between collisions with the walls / collisions with the walls become less frequent, compensating for the greater force per collision and maintaining constant pressure.
Key terms in this question
kinetic molecular model · pressure · constant pressure
Related
- All IB DP Physics Standard Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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