Question

Which of the following is correct for critical temperature?

• A. It is the highest temperature at which liquid and vapour can
  co-exist.
• B. Beyond this temperature, a gas cannot be liquefied by compression.
• C. At this temperature, the surface tension of the system is zero.
• D. At this temperature, the gas and the liquid phases have different critical densities.

Answer 1

Answer: (A), (B), (C)

The correct options are
A It is the highest temperature at which liquid and vapour can
co-exist.
B Beyond this temperature, a gas cannot be liquefied by compression.
C At this temperature, the surface tension of the system is zero.

At critical temperature $\left(T_c\right)$, the physical properties of liquid phase of substance is similar as physical properties of gaseous phase. At critical temperature both the phase of substance co-exist and we cannot distinguish between the two phases. Hence, density of liquid becomes same as density of gas at $T_c$ and surface tension vanishes. Above the critical temperature, gas can not be liquefied even if high pressure is applied.