Question

A gas can be liquefied :

• A. above its critical temperature
• B. at its critical temperature
• C. below its critical temperature
• D. at any temperature

Answer 1

Answer: (C)

below its critical temperature

Answer 2

According to kinetic theory of gases, the gases consists of tiny molecules widely separated from one another by large empty spaces (voids). Gases change to liquid when the voids between the gas molecules become less than $10^{-5} cm$. This can be achieved by decrease in temperature (as KE of molecules decrease) or by increase in pressure where in the molecules are brought close. However, it has been experimentally found that above a particular temperature known as critical temperature $\left(T_{c}\right)$, the liquification of a gas does not take place at all, howsoever high pressure may be applied. It is because above critical temperature of a gas, the kinetic energy of the gas molecules is sufficient enough to over-come attractive forces and keep the molecules apart. The pressure required to liquify the gas at critical temperature is called critical pressure $\left(P_{c}\right)$. The volume of one mole of a gas at $T_{c}$ and $P_{c}$ is called cirtical volume $\left(V_{c}\right)$. Thus a gas can be liquefied only below its critical temperature.