A student leaves a fully inflated balloon on a windowsill in a warm room. Later, after the room has cooled overnight, the student notices the balloon has shrunk in size. The amount of gas inside the balloon has not changed. Explain, using kinetic theory, why the balloon shrinks when the temperature decreases.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
The gas particles move more slowly when the temperature decreases.
The particles hit the walls of the balloon less frequently.
Each collision exerts a smaller force on the balloon wall.
The pressure exerted by the gas on the inside of the balloon decreases.
The lower internal gas pressure is less able to resist the elastic force of the balloon material, so the balloon contracts and its volume decreases.
The particles hit the walls of the balloon less frequently.
Each collision exerts a smaller force on the balloon wall.
The pressure exerted by the gas on the inside of the balloon decreases.
The lower internal gas pressure is less able to resist the elastic force of the balloon material, so the balloon contracts and its volume decreases.
Examiner tips
- Use the word "explain" to describe the chain of reasoning, not just state facts.
- Show the link between temperature, particle speed, collision frequency and pressure.
- Mention the balloon’s elastic force as the opposing factor.
- Keep the answer concise and use the exact terminology from the mark scheme.
Common mistakes
- Failing to mention the reduced collision frequency or force.
- Using vague terms like "less energy" instead of "lower kinetic energy".
- Omitting the role of the balloon’s elastic force in allowing contraction.
Mark scheme (5 marks)
- The gas particles move more slowly when temperature decreases
- The particles hit the walls of the balloon less frequently
- Each collision exerts a smaller force on the balloon wall
- The pressure exerted by the gas on the inside of the balloon decreases
- The lower internal gas pressure is less able to resist the elastic force of the balloon material, so the balloon contracts / volume decreases
Key terms in this question
Related
- All Eduqas A-Level Physics revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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