Question

At the critical temperature, the surface tension of a liquid
A) Is zero
B) Is infinity
C) Is the same as that at any other temperature
D) Can not be determined

Answer 1

Hint: The molecules of the fluid tend to have cohesive force each other. The surface tension can be termed as the property of the cohesive force of the fluid molecules to resist any change in the fluid.

Complete step by step answer:
We want to check how the surface tension changes with the temperature. The surface tension and the intermolecular force of the liquid molecules are related. If the molecules have high intermolecular force, then the surface tension will be larger. If this intermolecular bond between the molecules breaks it will decrease the surface tension.
Temperature is a major factor that affects this intermolecular bond between the molecules. When the temperature increases the molecules will gain kinetic energy which causes the movement of the molecules. Thus the liquid expands. Hence the intermolecular force gets weakened and reduces the surface tension. The liquid can no longer resist the change due to an increase in the temperature. The motion of the molecules will be rapid.

Surface tension decreases when the temperature increases. When it reaches the critical temperature, the surface tension will become zero. The correct answer is Option A.

Note:
The critical point is that the liquid and gas phases do not exist. It can be called as Fluids only. Surface tension is the property defined to liquid or gas phases. Therefore surface tension will be zero when it reaches a critical temperature.