Water boils at a lower temperature at high altitudes, because
- (a)the air pressure is less
- (b)outside temperature is less
- (c)latent heat is less
- (d)None of the above
Correct — A, the air pressure is less. A liquid boils at the temperature where its saturated vapour pressure becomes equal to the pressure pressing down on its surface. At sea level that outside pressure is about one atmosphere and water reaches it at 100 degrees Celsius. Climb a mountain and the column of air above you is shorter, so atmospheric pressure falls; water's vapour pressure now matches the surroundings at a lower temperature, and the water boils sooner. The same physics run backwards inside a pressure cooker, where trapped steam raises the pressure and water boils at about 121 degrees Celsius instead.
- (b)outside temperature is less — Cold surroundings mean the water takes longer to get hot; they do not change the temperature at which it boils. Boil water in a sealed laboratory at sea level on a freezing day and it still boils at 100 degrees Celsius.
- (c)latent heat is less — This inverts the relationship. The latent heat of vaporisation is a property of the liquid that varies slightly with the boiling temperature — it is a consequence, not the cause, and reduced pressure is what does the causing.
- (d)None of the above — Option (a) states the standard and correct explanation, so a blanket rejection cannot stand.
Molecules escape from a liquid surface and build up a vapour above it. The pressure that vapour exerts rises steeply with temperature. Boiling begins the moment the vapour pressure equals the external pressure, because only then can bubbles of vapour form inside the body of the liquid and survive the squeeze of the surroundings. That is why boiling is a bulk phenomenon, happening throughout the liquid, while evaporation happens only at the surface and at any temperature.
Two everyday consequences make the rule easy to recall. On a high mountain, water boils well below 100 degrees Celsius, and food cooked in it stays undercooked however long it bubbles, because cooking depends on temperature and not on the sight of boiling. In a pressure cooker the steam is confined, the pressure rises by roughly one bar above the outside, and the water boils at about 121 degrees Celsius, so food cooks faster. Both are the same equation read in opposite directions. Note also a favourite trap of examiners: a well-known UPSC assertion-reason item paired the true statement that boiling point falls with altitude with the false claim that atmospheric pressure rises with altitude.
- A liquid boils when its saturated vapour pressure becomes equal to the external pressure on its surface.
- Atmospheric pressure falls with altitude, so the boiling point of water falls with it.
- A domestic pressure cooker runs at a gauge pressure of about 0.8 to 1 bar, at which water boils at about 121 degrees Celsius.
- Boiling occurs throughout the bulk of a liquid at a fixed temperature; evaporation is a surface process that happens at any temperature.
- Food cooked at a lower boiling temperature stays undercooked no matter how long it boils, because the cooking depends on temperature.
Raise the pressure in a cooker and the boiling point climbs to about 121 degrees Celsius; drop it on a mountain and the boiling point falls.
- Confusing the low outside temperature on a mountain with the cause of the low boiling point.
- Assuming faster boiling means faster cooking; the temperature of the water is what cooks the food.
- Believing atmospheric pressure increases with altitude — a UPSC assertion-reason item was built on exactly that error.
NDA asks for the condition that defines boiling, or for the reason behind a pressure cooker, a mountain kitchen or a vacuum flask.
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
The identical fact set as an assertion-reason item, with the trap moved into the reason — pressure falls with altitude, it does not rise.
A liquid is heated up to a certain temperature. Which one of the following situation would correspond to the boiling of the liquid ?
- (a) When atmospheric pressure becomes equal to the vapour pressure
- (b) When atmospheric pressure becomes less than vapour pressure
- (c) When atmospheric pressure becomes higher than the vapour pressure
- (d) When vapour pressure becomes equal to the air pressure
Answer(a) When atmospheric pressure becomes equal to the vapour pressure
States the underlying rule that this question applies to a mountain: boiling begins where vapour pressure meets the external pressure.
- practice — not a real PYQ
Food takes longer to cook properly on a high mountain mainly because
- (a)the flame is weaker in thin air
- (b)water boils at a temperature below 100 degrees Celsius there
- (c)the latent heat of water is greater at high altitude
- (d)heat escapes faster from the vessel
Answer(b) water boils at a temperature below 100 degrees Celsius there — the lower boiling temperature, not the boiling itself, is what leaves the food undercooked.
- practice — not a real PYQ
Which one of the following statements about boiling and evaporation is correct ?
- (a)Both are surface phenomena
- (b)Boiling is a surface phenomenon and evaporation a bulk phenomenon
- (c)Boiling is a bulk phenomenon and evaporation a surface phenomenon
- (d)Both are bulk phenomena
Answer(c) Boiling is a bulk phenomenon and evaporation a surface phenomenon — bubbles form throughout the liquid only when vapour pressure matches the external pressure.