Question

The highest temperature at which vapour pressure of a liquid can be measured is:
A. Critical temperature
B. Inversion temperature
C. Critical solution temperature
D. Boiling point of water

Answer 1

When a liquid is heated at its highest temperature, liquid converts into gas. The temperature at which the vapour pressure of the liquid approaches the atmospheric pressure, the vapour pressure of the liquid can be measured.

Complete answer:
A. Critical temperature is the temperature, below which a gas can be liquefied. As the gas converts into a liquid the vapour pressure decreases so, it is not the highest temperature at which vapour pressure of a liquid can be measured.

B. Inversion temperature is the temperature, below which if the gas is allowed to expand it shows the cooling effect and above the inversion temperature gas shows the heating effect so, it is not the highest temperature at which vapour pressure of a liquid can be measured.

C. Critical solution temperature is the temperature at which two phases coexist so, it is not the highest temperature at which vapour pressure of a liquid can be measured.

D. The boiling point is the temperature at which the vapour pressure of the liquid becomes equal to the vapour pressure of the atmosphere, so the liquid boils and converts into the gas from liquid. During boiling complete liquid boils and convert into the gas phase. So, now the pressure applied by the vapour on the liquid cannot be measured because there is no liquid state on which the vapour can apply pressure.
So, the highest temperature at which vapour pressure of a liquid can be measured is the boiling point.

**Therefore, option (D) Boiling point of water, is correct.

Note:**
Vapour pressure is the pressure excreted by the vapour on the liquid or solid. As a liquid is heated, its temperature increases. On increasing the temperature, the liquid starts to convert into vapour, so vapour pressure increases. So, the vapour pressure can be measured. At a temperature, the vapour pressure of the liquid becomes equal to the vapour pressure of the atmosphere. At this temperature liquid completely converts into vapour so the vapour pressure can be measured till the boiling point. The formula of inversion temperature is ${{\text{T}}_{\text{i}}}\,{\text{ = }}\,\dfrac{{{\text{2a}}}}{{{\text{Rb}}}}$. The formula of critical temperature is ${{\text{T}}_{\text{C}}}\,{\text{ = }}\,\dfrac{{\text{a}}}{{{\text{Rb}}}}$.