UPSC Prelims 2003 Q8: Science & Technology – Physics
A hydrogen - inflated polythene balloon is released from the surface of the earth. As the
balloon rises to an altitude up in the atmosphere, it will
- (a) Decrease in size
- (b) Flatten into a disc - like shape
- (c) Increase in size
- (d) Maintain the same size and shape
Answer: (c) Increase in size
Explanation
A rising hydrogen balloon
| Altitude | Effect |
|---|---|
| Higher | Atmospheric pressure falls |
| Gas inside | Expands against the lower external pressure |
| Balloon | Increases in size, until it bursts if the skin cannot stretch |
As the balloon rises, the outside pressure decreases, so the gas inside expands and the balloon increases in size.
Answer: (c) Increase in size.
Why each option is right or wrong
- (a) Incorrect. Lower outside pressure makes it expand, not shrink.
- (b) Incorrect. It would not flatten into a disc.
- (c) CORRECT. The balloon expands as the pressure falls.
- (d) Incorrect. The size does not stay the same.
Key point
Boyle's law: lower pressure, larger volume at constant temperature.
Remember
A rising balloon expands because the outside pressure falls.
Common mistakes
- Thinking low temperature at height shrinks the balloon more.
- Forgetting that pressure drops with altitude.
- Confusing a rising balloon with a descending one.
Exam tip
Weather balloons expand and finally burst at great heights.
Sources
- NCERT Class XI Physics — Thermal Properties of Matter (gas laws)
- NCERT Class XI Chemistry — States of Matter (gas laws)