Question

In most liquids, with rise in temperature surface tension of liquid

• A. decreases
• B. increases
• C. remains unchanged
• D. first decreases and then increases

Hint:

Higher temperature weakens the cohesive forces between liquid molecules.

Answer 1

The Correct Option is A

Step 1: Understand the concept
Surface tension arises from the cohesive forces between molecules at the surface of a liquid. It is a measure of the energy needed to increase the surface area.

Step 2: Effect of temperature
On heating, molecules move faster and move further apart, so the average cohesive force between them weakens.

Step 3: Result
With weaker cohesion, less energy is needed to create surface, so the surface tension decreases as the temperature rises. It becomes zero at the critical temperature, where the boundary between liquid and vapour disappears.

Step 4: Options
The surface tension does not increase (B), stay constant (C), or rise again after falling (D) for most liquids. The answer is option (A).

Final Answer:
Surface tension decreases as temperature rises. This is option (A). \[ \boxed{\text{(A) }\text{Decreases}} \]