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Byju's Answer
Standard XII
Physics
Boiling
A liquid boil...
Question

A liquid boils when its vapour pressure equals _________.

A
the atmosphere pressure
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B
the pressure of 76.0 cm column of mercury
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C
the critical pressure
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D
the dew point of the surroundings
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Solution

The correct option is A the atmosphere pressure
A liquid gets converted into vapor phase when its vapor pressure becomes equal to the atmospheric pressure.
That's why, a liquid boils faster at high altitude than sea level.

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Q. A liquid boils when vapour pressure becomes equal to atmospheric pressure.
Q.
Comprehension # 2
Following passage explains the effect of temperature on the vapour pressure of the liquid.

Effect of temperature on vapour pressure
The quantity of heat required to evaporate a given liquid at constant temperature is defined as the heat of vaporisation. Variation of vapour pressure with temperature is given by,

Clausius-Clapeyron equation.

logeP=−ΔHvapRT+logeA

A liquid is said to be at its boiling temperature if its vapour pressure is equal to external pressure. Therefore, the boiling point of water in particular and of liquids in general decreases as an altitude of a place increases where the external pressure is less than 1atmosphere (normal b.p. of water is 373.15K at 1atmosphere)

On top of mount everest, for example, where the atmospheric pressure is only about 260mmHg, water boils at approximately 71∘C. Conversely, if the external pressure on a liquid is greater than 1atm, the vapour pressure necessary for boiling than normal boiling is reached later, and the liquid boils at a temperature greater than the normal boiling point.

Clausius-Clapeyron equation can be written in the following form:

Q.

The water boils when :

1.Saturated vapour pressure of water becomes equal to atmospheric pressure

2. Boiling point of water becomes more than atmospheric pressure

3. Saturated vapour pressure of water is less than atmospheric pressure

4. Vapour pressure of water becomes more than atmospheric pressure

Q.

When the vapour pressure of a liquid is equal to its atmospheric pressure, then it


Q. The vapour pressure of a liquid at its boiling point is equal to ___ atm.
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