Question

A gas is liquefied
a.) Above critical temperature and below critical pressure.
b.) Below critical temperature and above critical pressure.
c.) Below critical temperature and pressure.
d.) Above critical temperature and pressure.

Answer 1

Hint: Liquefaction of a gas is based on the various concepts like Joule-Thomson effect, and compressing the gas. It is related to the critical temperature and pressure. The compression of gas takes place by increasing the pressure.

Complete step by step answer:
First, let us know about the liquefaction of a gas. It occurs when the molecules of a gas are pushed closer.
If we talk about the squeezing of gas molecules, it is done by increasing the pressure on the gas, and lowering the temperature of the gas.
The terms critical pressure and critical temperature can be defined with the help of Joule-Thomson effect. The critical temperature is temperature above which the gas cannot be liquefied by pressure alone.
Now, if we want to convert a gas into liquid, we have to decrease the temperature below the critical temperature because if gas works against an external force, it starts losing energy and converts into the liquid state.
As we mentioned about the critical temperature, therefore we have to increase the pressure above the critical pressure.
So, we can conclude that a gas is liquefied below critical temperature and above critical pressure. The correct option is B.

Note: Don’t get confused between the temperature and critical temperature. These are different terms. If you increase the temperature, and decrease the pressure; the whole process comes back to the original one.