When the temperature of a liquid increases, its surface tension

Analysis of the Question

This question tests your understanding of the molecular behavior of liquids and the concept of surface tension in relation to thermal energy.

Concept Explanation

Surface tension is the property of a liquid surface that allows it to resist an external force, caused by the cohesive forces between liquid molecules.

Think of it this way: molecules at the surface of a liquid are pulled inward by the molecules beneath them. This creates a "skin" on the surface. When you increase the temperature of a liquid, you are providing kinetic energy to the molecules. As the molecules move faster and vibrate more vigorously, the cohesive forces (the "attraction" holding them together) are weakened. Because these cohesive forces are responsible for surface tension, weakening them leads to a reduction in surface tension.

Step-by-Step Solution

1. Identify the relationship: Surface tension is directly dependent on the strength of the cohesive forces between molecules.
2. Apply the effect of heat: Increasing the temperature increases the average kinetic energy of the molecules.
3. Observe the result: The increased kinetic energy allows molecules to overcome the cohesive forces more easily, causing the liquid to expand and the surface tension to drop.
4. Conclusion: As temperature rises, the cohesive forces weaken, and therefore, the surface tension decreases.

Correct Option

The correct option is A.

Exam Tip

To remember this easily, think of hot water vs. cold water. Hot water is a better cleaning agent than cold water because its lower surface tension allows it to "wet" surfaces better and penetrate fabrics more easily. Therefore, Heat = Higher Temperature = Lower Surface Tension.