UPSC Prelims 2001 Q103: Science & Technology – Physics
Assertion (A): The boiling point of water decreases as the altitude increases.
Reason (R): The atmospheric pressure increases with altitude.
- (a) Both A and R are individually true and R is the correct explanation of A
- (b) Both A and R are individually true but R is NOT a correct explanation of A
- (c) A is true but R is false
- (d) A is false but R is true
Answer: (c) A is true but R is false
Explanation
Assertion-Reason: boiling point of water at high altitude
| Statement | Verdict |
|---|---|
| A. The boiling point of water falls as altitude rises | True — about 1 °C lower for every 300 m |
| R. Atmospheric pressure increases with altitude | False — pressure decreases with altitude |
| Real reason for A | Water boils when its vapour pressure equals the air pressure; lower air pressure means a lower boiling point |
| Example | On Everest water boils at about 70 °C |
Water boils when its vapour pressure equals the pressure of the air above it. At height the air pressure is lower, so water boils at a lower temperature. R states the opposite of the truth, so A is true and R is false.
Answer: (c) A is true but R is false.
Why each option is right or wrong
- (a) Incorrect. R is false; pressure falls with altitude.
- (b) Incorrect. R is false, so both cannot be true.
- (c) CORRECT. Boiling point falls with height, but pressure falls too, not rises.
- (d) Incorrect. A is true; water boils at lower temperatures in the hills.
Key point
A pressure cooker raises pressure to about 2 atmospheres, so water boils at about 120 °C and food cooks faster.
Remember
Altitude up → pressure down → boiling point down.
Common mistakes
- Reading R quickly and missing 'increases'.
- Thinking it is cold at height, so water boils later.
- Not linking boiling point to air pressure.
Exam tip
Higher pressure → higher boiling point; lower pressure → lower boiling point.
Sources
- NCERT Class IX — Science, chapter 'Matter in Our Surroundings'
- NCERT Class XI — Chemistry Part I, chapter 'States of Matter'