Surface tension of a liquid is a molecular phenomenon of liquids involving the force of cohesion among the liquid molecules. It is a scalar quantity and numerically equal to the surface energy. Numerically, it is proved that the potential energy of a liquid is maximum on the surface. Sparingly soluble salts and surface active substances decrease the surface tension of the liquid, however, the fairly soluble solute increase is independent of surface area but if depends on the intermolecular force and the temperature.
Surface tension of a liquid is zero at:

Options

A. inversion temperature
B. boiling point
C. critical point
D. saturation point.

Explanation

To solve the question regarding the surface tension of a liquid, we need to understand the concept of surface tension and its behavior under various conditions. Here’s a step-by-step solution:

### Step 1: Understand Surface Tension
Surface tension is a property of liquids that arises from the cohesive forces between liquid molecules. It is a measure of the elastic tendency of a fluid surface and is responsible for the shape of liquid droplets.

**Hint:** Surface tension is related to the cohesive forces among molecules in a liquid.

### Step 2: Identify the Nature of Surface Tension
Surface tension is a scalar quantity and is numerically equal to the surface energy per unit area. It is influenced by factors such as temperature and the presence of solutes.

**Hint:** Remember that surface tension can change with temperature and the addition of certain substances.

### Step 3: Consider the Effect of Temperature
As the temperature of a liquid increases, the kinetic energy of the molecules also increases. This leads to a decrease in cohesive forces among the molecules, thereby reducing surface tension.

**Hint:** Think about how heating a liquid affects the movement of its molecules.

### Step 4: Determine the Critical Point
The critical point of a substance is the temperature and pressure at which the distinction between the liquid and vapor phases disappears. At this point, the surface tension becomes zero because the liquid and vapor phases are indistinguishable.

**Hint:** The critical point is where the properties of liquid and gas converge.

### Step 5: Conclusion
Therefore, the surface tension of a liquid is zero at its critical point. This is the temperature at which the liquid can no longer exist as a distinct phase from its vapor.

**Final Answer:** The surface tension of a liquid is zero at the critical point.

### Summary of Steps:
1. Understand the concept of surface tension.
2. Recognize that it is a scalar quantity related to cohesive forces.
3. Note that temperature affects surface tension.
4. Identify the critical point where liquid and vapor phases merge.
5. Conclude that surface tension is zero at the critical point.