Question

If T and \(\eta\) are the surface tension and coefficient of viscosity of a liquid, then with the increase of temperature

• A. both T and \(\eta\) increase
• B. both T and \(\eta\) decrease
• C. both T and \(\eta\) remain constant
• D. T increases but \(\eta\) decreases
• E. T decreases but \(\eta\) increases

Hint:

Hot water has lower surface tension (better for cleaning) and lower viscosity (flows more easily) than cold water.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
This question explores how two key properties of liquids, surface tension and viscosity, are affected by changes in temperature. Both properties are related to the intermolecular forces within the liquid.
Step 2: Detailed Explanation:

Effect of Temperature on Surface Tension (T): Surface tension is the tendency of liquid surfaces to shrink into the minimum surface area possible. It arises from the cohesive intermolecular forces among the liquid molecules. When the temperature of a liquid is increased, the average kinetic energy of its molecules increases. This increased movement of molecules tends to weaken the cohesive forces between them. As the intermolecular forces decrease, the surface tension also decreases. At the boiling point, the surface tension of a liquid becomes zero.
Effect of Temperature on Viscosity (\(\eta\)) of a Liquid: Viscosity is a measure of a fluid's resistance to flow. In liquids, viscosity is primarily caused by the cohesive forces between molecules that create an internal friction. Similar to surface tension, when the temperature of a liquid increases, the kinetic energy of the molecules increases, which overcomes the intermolecular cohesive forces. This allows the liquid layers to slide past one another more easily, resulting in a decrease in viscosity.
Therefore, for a liquid, both surface tension and the coefficient of viscosity decrease as the temperature increases.
(Note: The original question used 'n' for viscosity, which has been corrected to the standard symbol \(\eta\)).
Step 3: Final Answer:
With an increase in temperature, both surface tension (T) and viscosity (\(\eta\)) of a liquid decrease. This corresponds to option (B).