The temperature at which "vapour pressure of the liquid in an open vessel becomes equal to the atmospheric pressure" is called
- (a)Melting point
- (b)Boiling point
- (c)Liquid point
- (d)None of the above
Correct — B, boiling point. A liquid boils when its saturated vapour pressure rises to equal the pressure pushing down on its surface; in an open vessel that is the atmospheric pressure. At exactly that temperature vapour bubbles can form throughout the bulk of the liquid — not just evaporate from the surface — and rise freely, so the whole liquid boils.
- (a)Melting point — The melting point is the temperature at which a solid turns into a liquid; it is fixed by the solid-liquid transition and has nothing to do with vapour pressure matching the atmospheric pressure.
- (c)Liquid point — 'Liquid point' is not a defined physical quantity — there is no such term in thermodynamics. It is a made-up distractor.
- (d)None of the above — Wrong because option (b), boiling point, exactly fits the definition given, so 'none of the above' cannot be correct.
The boiling point of a liquid is the temperature at which its saturated vapour pressure becomes equal to the external pressure above it. In an open vessel that external pressure is the atmospheric pressure, so the boiling point depends on how high or low that pressure is.
The trap is to confuse the boiling point (liquid to vapour) with the melting point (solid to liquid). Anchor on the wording 'vapour pressure equal to atmospheric pressure' — that is the textbook definition of boiling. Because it is tied to the surrounding pressure, water boils below 100 degrees Celsius on a high mountain and above 100 degrees Celsius inside a pressure cooker.
- A liquid boils when its vapour pressure equals the surrounding pressure; in an open vessel that is the atmospheric pressure.
- At standard atmospheric pressure (101.3 kPa) pure water boils at 100 degrees Celsius (373 K).
- Lower external pressure lowers the boiling point, so water boils below 100 degrees Celsius at high altitude.
- A pressure cooker raises the internal pressure, so water boils above 100 degrees Celsius and food cooks faster.
- Confusing boiling point (liquid to vapour) with melting point (solid to liquid).
- Forgetting that the boiling point changes with external pressure — it is not a fixed 100 degrees Celsius everywhere.
Asked either as this definition, or as 'why does water boil below 100 degrees Celsius on a hill and above 100 degrees Celsius in a pressure cooker'.
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
Same concept — boiling occurs when vapour pressure equals the surrounding pressure. There the boiling point falls with altitude because atmospheric pressure drops (so the reason, which claims pressure rises, is false); here the boiling point is defined as the temperature at which vapour pressure equals the atmospheric pressure.
- practice — not a real PYQ
Water boils at a temperature lower than 100 degrees Celsius on a high mountain because
- (a)the air is colder there
- (b)the atmospheric pressure is lower there
- (c)the water is purer there
- (d)gravity is weaker there
Answer(b) the atmospheric pressure is lower there — so the vapour pressure equals it at a lower temperature.
- practice — not a real PYQ
A pressure cooker cooks food faster mainly because
- (a)steam has more heat than boiling water
- (b)the raised internal pressure increases the boiling point of water above 100 degrees Celsius
- (c)it seals in flavour
- (d)the metal conducts heat quickly
Answer(b) the raised internal pressure increases the boiling point above 100 degrees Celsius, so food cooks at a higher temperature.