A sealed metal container holds a fixed quantity of gas. The container is placed in boiling water, causing the temperature of the gas to rise. Explain how and why the pressure of the gas changes as its temperature increases.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A sealed metal container holds a fixed quantity of gas at room temperature. The container is then placed in boiling water.
Model answer (5 marks)
The temperature of the gas rises, so the average kinetic energy of the gas molecules increases.
The molecules therefore move faster and have greater velocities.
They collide with the walls of the container more frequently.
Each collision exerts a greater force on the walls.
Thus the pressure of the gas increases, because pressure is the perpendicular force per unit area on the walls and the volume (area) stays constant.
The molecules therefore move faster and have greater velocities.
They collide with the walls of the container more frequently.
Each collision exerts a greater force on the walls.
Thus the pressure of the gas increases, because pressure is the perpendicular force per unit area on the walls and the volume (area) stays constant.
Examiner tips
- Use the kinetic theory chain: temperature → kinetic energy → velocity → collision frequency → force → pressure.
- Show the final conclusion that pressure rises because volume is fixed.
- Use the exact phrase "perpendicular force per unit area" to hit the mark.
- Mention that the container is sealed so the volume does not change.
Common mistakes
- Confusing temperature with pressure; forgetting that the volume is constant.
- Using vague terms like "more gas" instead of "more kinetic energy".
- Omitting the step that collisions become more frequent and stronger.
Mark scheme (5 marks)
- The temperature of the gas increases, so the average kinetic energy of the gas molecules increases.
- The molecules move faster / have greater velocities.
- The molecules collide with the walls of the container more frequently.
- The molecules collide with the walls with greater force.
- Therefore the pressure of the gas increases, because pressure is the perpendicular force per unit area on the walls and the volume (area) stays constant.
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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