Question

Which one of the following statements is correct about critical temperature?
A. It is the temperature at which, saturated vapor pressure of a liquid equals the atmospheric pressure
B. It is the temperature below which, evaporation of the liquid occurs very rapidly
C. It is the temperature above which a gas cannot be liquefied.
D. It is the temperature at which the states of solid, liquid and gas co- exist.

Answer 1

Hint: Here we should understand the concept of critical temperature, Critical temperature is the maximum temperature at which liquid and vapor can coexist. Beyond this temperature, there is no distinction between the two phases.

Step by step answer: Critical temperature is the temperature at which gas cannot exist liquefied does not matter what pressure be applied. At critical temperature, the properties of gas and liquid evaporates and heat of vaporization beyond this point becomes 0.

The critical temperature is the highest temperature at which liquid and vapor can coexist each other.
Beyond the critical temperature, there is no difference between the two phases and a gas cannot be liquefied by a compression process.

At a critical temperature the gas and the liquid phases have different critical densities.

We can explain this with the help of a graph describing the triple point and the critical point of a substance. It can be noted that the graph is plotted with pressure on the Y-axis and temperature on the X-axis.
The critical pressure of a substance is the pressure that must be applied in order to liquefy that substance at its critical temperature. For example, 217.7 atmospheres of pressure must be applied to water in order to liquefy it at its critical temperature.

Hence the correct option is C.

Note: In thermodynamics, a critical point is the end point of a phase equilibrium curve. The most prominent example is the liquid–vapor critical point, the end point of the pressure–temperature curve that designates conditions under which a liquid and its vapor can coexist