At the critical temperature, the surface tension of a liquid is:-
C.Varies liquid to liquid
D.Cannot be measured.

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Surface tension is the characteristics property of the liquids. The property is caused due to the strong intermolecular forces of attraction between the liquid molecules. It is defined as the force acting per unit length perpendicular to the line drawn on the surface of the liquid.

Complete answer:
As the temperature increases, surface tension decreases. This is because, with the rise in temperature the kinetic energy of the liquid molecules increases. This in turn decreases the intermolecular forces. It results in a decrease of inward pulling on the surface of the liquid. So the surface tension decreases with a rise in temperature and as the temperature approaches critical temperature the surface tension becomes zero.
A.ZERO:- As the critical temperature approaches the surface tension becomes zero because the intermolecular forces between particles become zero. Hence, the option is correct.
B.Infinite:- due to critical temperature the intermolecular forces become zero so surface tension decreases. Molecules move freely. Hence the option is incorrect.
C. Varies liquid to liquid.:- it only depends on the force of attraction between liquid so as compared to water honey will have more surface tension. But at critical temp-any type of liquid shows zero surface tension. So the option is incorrect.
D.Can not be measured:- it can be measured as it is the measure of attractive forces and temperature. So the option is wrong.

Our required Ans is zero.

The spherical shape of drops is due to surface tension. The effect of surface tension is to reduce the area of the surface to a minimum. As the sphere has a minimum surface area for a given volume of a liquid, so, the drops of liquid-like mercury or bubbles of gas are spherical in shape.
The inward forces on the surface molecule minimize the surface area and form a drop.