Which of the following is NOT TRUE?
a. Any increase in temperature of liquid decreases its angle of contact for a given solid-liquid pair.
b. When a soluble substance is dissolved in water, the surface tension of water decreases.
c. When a sparingly soluble substance is mixed with water, the surface tension of water decreases.
d. When insoluble impurity is added into water, surface tension of water decreases.

The angle of contact generally decreases with an increase in temperature for a given solid-liquid pair.

When a highly soluble impurity (like sodium chloride or sugar) is dissolved in water, the intermolecular forces of attraction increase, which results in an increase in the surface tension of water. Therefore, statement b is incorrect.

When a sparingly soluble substance (like soap, detergent, or phenol) is mixed with water, the surface tension of water decreases.

When an insoluble impurity (like dust or oil) is added to water, the cohesive forces between water molecules are weakened, which decreases the surface tension of water.

Answer: b