Question

Which of the following is / are correct for critical temperature?

• A. It is the highest temp. at which liquid and vapour can coexist
• B. Beyond the critical temperature, there is no distinction between two phases and gas can not be liquiefied by compression
• C. At critical temperature, the surface tension of the system is zero
• D. At critical temperature, the gas and the liquid phases have the different critical densities

Answer 1

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

The correct options are
A It is the highest temp. at which liquid and vapour can coexist
B Beyond the critical temperature, there is no distinction between two phases and gas can not be liquiefied by compression
C At critical temperature, the surface tension of the system is zero

The critical temperature of a substance is the temperature above which vapour of the substance cannot be liquified, no matter how much pressure is applied.
At the critical point, the liquid and vapour phases have exactly the same density and only a single phase exists.
Above the critical temperature liquid can't be formed. So, it is the highest temperature where liquid and vapour can co-exist.
So, option (D) is incorrect and option (a) and (b) are correct.

It is known that surface tension decreases with the increase of temperature reaching a value of zero at the critical temperature.
The option (C) is also correct.