Question

Surface tension decreases with increase in ___.

• A. temperature
• B. addition of surface active solutes
• C. none of these

Answer 1

Answer: (A), (B)

The correct options are
A temperature
B addition of surface active solutes

So, here we are talking about any factor by which surface tension decreases.
Let's check both the factors one by one.

Effect of temperature on surface tension: The magnitude of the surface tension of a liquid depends on the attractive forces between the molecules. When the attractive forces are large, the surface tension is large. An increase in temperature increases the kinetic energy of the molecules, and the effectiveness of intermolecular attraction decreases. So the surface tension decreases as the temperature is raised.

Effect of adding surface active solutes: The addition of surface active solutes to a liquid also lowers the surface tension. For example, the addition of ethanol, soaps, detergents, etc., to water lowers its surface tension. The cleansing action of soap is based on this fact.
You will learn more about this in surface chemistry.