Question

The temperature at which the vapour pressure of a liquid becomes equal to the external (atmospheric) pressure is its
A. Inversion point
B. Melting point
C. Boiling point
D. None of these

Answer 1

Hint: Vapourization is the phase change from liquid to vapour state.
After vaporisation, at equilibrium, the vapours exert pressure over the liquid phase which is called vapour pressure.

Complete Step by Step Solution:
When the liquid state gets transformed into a vapour state, the process is known as vaporisation.
Liquids at a temperature undergo vaporisation.
At equilibrium, the pressure exerted by the vapours of the liquid over the liquid state is known as the vapour pressure. It increases with the increase in temperature. The boiling of a liquid happens at a temperature at which its vapour pressure is equivalent to the atmospheric pressure. This temperature at which the vapour pressure of a liquid becomes equal to the external or atmosphere pressure is called boiling point.
For example:- Boiling of water happens at 373K as at this temperature the vapour pressure of water is 1 atmosphere.
So, option C is correct.

Additional Information: In a pure liquid, the entire surface is maintained by the molecules of the liquid. If a non-volatile solute is added to a solvent, then the surface of the liquid possesses both solute and solvent molecules. The fraction of the surface enclosed by the solvent molecules is decreased. Accordingly, the number of solvent molecules escaping from the surface also decreases. So, vapour pressure decreases. With the decrease in vapour pressure, more temperature is required to surpass the external pressure so that the liquid will be able to attain the vapour state. Hence, the boiling point increases called elevation in boiling point.

Note: Vapour pressure increases with temperature. With the increase in temperature, the kinetic energy of the liquid particles increases. As a result, more liquid particles possess a sufficient amount of energy required to attain the gas phase. So, when the vapour pressure becomes equal to the external pressure, boiling happens.