A weather balloon is filled with a fixed mass of gas at ground level, where the temperature is 17 °C and the pressure is high. As the balloon rises into the upper atmosphere, the surrounding pressure decreases and the temperature drops to −33 °C. Explain what happens to the volume of the gas inside the balloon as it rises, considering separately the effect of decreasing pressure and the effect of decreasing temperature.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Weather balloons are used to carry scientific instruments high into the atmosphere. The balloon is only partially inflated at ground level before release.
Model answer (5 marks)
The gas is initially at 17 °C and high pressure. As the balloon rises the external pressure falls.
1. With temperature constant, Boyle’s law (P V = constant) gives V∝1/P – the lower pressure allows the gas to expand, so the volume increases.
2. The temperature falls to –33 °C. By Charles’ law (V∝T) the lower temperature would reduce the volume, because the gas molecules move more slowly and exert less pressure on the walls.
3. The pressure effect is much larger than the temperature effect; the decrease in external pressure dominates, so the net change is an increase in volume.
4. Consequently the balloon expands as it rises, reaching a larger volume at the higher altitude.
5. The balloon will continue to expand until the internal pressure equals the external pressure or the material limits are reached.
1. With temperature constant, Boyle’s law (P V = constant) gives V∝1/P – the lower pressure allows the gas to expand, so the volume increases.
2. The temperature falls to –33 °C. By Charles’ law (V∝T) the lower temperature would reduce the volume, because the gas molecules move more slowly and exert less pressure on the walls.
3. The pressure effect is much larger than the temperature effect; the decrease in external pressure dominates, so the net change is an increase in volume.
4. Consequently the balloon expands as it rises, reaching a larger volume at the higher altitude.
5. The balloon will continue to expand until the internal pressure equals the external pressure or the material limits are reached.
Examiner tips
- Use the correct gas laws (Boyle’s and Charles’) and state the proportionality clearly.
- Explain the opposing effects of pressure and temperature before giving the net result.
- Show the reasoning in a logical order – first pressure, then temperature, then conclusion.
Common mistakes
- Confusing temperature with pressure in the explanation of volume change.
- Failing to mention that the temperature effect opposes the pressure effect.
- Giving only one effect (pressure or temperature) and ignoring the other.
Mark scheme (5 marks)
- As pressure decreases (at constant temperature), the gas molecules hit the walls of the balloon less frequently per unit area / the walls are pushed outward more / volume increases
- Pressure and volume are inversely proportional (for a fixed mass of gas at constant temperature), so lower pressure means larger volume
- As temperature decreases, the gas molecules move more slowly / have less kinetic energy
- Slower-moving molecules exert less force/pressure on the walls, so the volume decreases (opposing the pressure effect)
- Overall the volume increases because the effect of the large pressure decrease dominates over the effect of the temperature decrease / the balloon expands as it rises
Key terms in this question
pressure · volume · temperature · fixed mass of gas
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