Question

Assertion: At $1{\text{ atm}}$, pure water can boil at a temperature less than $273K$.
Reason: Water boils when the vapour pressure of the water is equal to the atmospheric pressure.
A.Both assertion and reason are true and reason is the correct explanation of assertion.
B.Both assertion and reason are correct and reason is not the correct explanation of assertion
C.Assertion is true but reason is false
D.Assertion is false but reason is true

Answer 1

Hint: To answer this question you must recall Boyle's law. A liquid vaporizes when the vapour pressure of the liquid becomes equal to the pressure of the air above the surface of water.

Complete step by step answer:
The Boyle’s law gives the relation between the exerted pressure of the gas and the change occurring in its volume as a result while maintaining a constant temperature. It states that the pressure exerted by a given quantity of a gas is inversely proportional to the volume that the gas occupies under constant conditions of temperature for a closed system. Thus we can also say that, ${\text{PV = constant}}$.
Using Boyle's Law, we can say that air is less dense at high altitudes, because the atmospheric pressure is lower. If pressure is increased, the gases become denser. Thus, it can be said that the density of a gas is directly proportional to the pressure.
At $1{\text{ atm}}$ pressure, the boiling point of water is equal to ${100^o}C$ or $373K$. Thus the assertion is false.
We know that liquids vaporize when the vapour pressure of water becomes equal to the atmospheric pressure.
Thus, we can say that Assertion is false but the reason is true.

Hence, the correct option is D.

Note:
The vapour pressure is the pressure exerted by a gas when a certain number of particles that leave the liquid is equal to the number of particles entering the liquid. As the temperature is increased, more particles attain energy enough to escape into the gaseous phase. Thus, the vapour pressure increases on heating and eventually becomes equal to the atmospheric pressure causing the liquid to boil.