A weather balloon is filled with gas at ground level. As the balloon rises to high altitude, the temperature of the gas drops significantly. Explain, in terms of gas particles, why the pressure of the gas inside the balloon decreases as the temperature falls. Assume the volume of the balloon stays constant.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Weather balloons carry instruments into the upper atmosphere, where temperatures can be extremely low.
Model answer (4 marks)
Decreasing temperature reduces the kinetic energy of the gas particles, so they move more slowly. Consequently, they collide with the balloon walls less often and each collision exerts a smaller force because the particles are moving more slowly. Pressure is the perpendicular force per unit area on the walls; with fewer and weaker collisions, the pressure inside the balloon decreases.
Examiner tips
- Use the term ‘kinetic energy’ and ‘collisions’ to show understanding of the kinetic theory.
- Explain both the frequency and force of collisions to cover all four points.
- Mention that the volume is constant to justify the pressure change is due to temperature only.
Common mistakes
- Saying the pressure increases instead of decreases.
- Confusing temperature with volume changes.
- Using vague phrases like ‘the gas gets colder’ without linking to kinetic energy.
Mark scheme (4 marks)
- Decreasing temperature means the gas particles have less kinetic energy / move more slowly
- Particles collide with the walls of the balloon less frequently
- Each individual collision exerts less force on the walls because the particles are moving more slowly
- Pressure is the perpendicular force per unit area on the container walls, so with less frequent and less forceful collisions, the pressure decreases
Key terms in this question
pressure · temperature · particles
Related
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