Question

By adding soluble impurity in a liquid, angle of contact

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

Hint:

Physics Tip: Detergents and soaps are common examples of soluble impurities (surfactants) added to water to decrease its surface tension and contact angle, which significantly improves its ability to wet and clean surfaces.

Answer 1

The Correct Option is A

Concept: Physics (Properties of Fluids) - Surface Tension and Contact Angle.

Step 1: Understand the relationship between surface tension and contact angle. The contact angle ($\theta$) is determined by the balance of interfacial tensions between the solid, liquid, and gas phases, as described by Young's equation: $$ \cos(\theta) = \frac{\sigma_{SG} - \sigma_{SL}}{\sigma_{LG}} $$ where $\sigma_{LG}$ represents the surface tension of the liquid-gas interface.

Step 2: Analyze the effect of soluble impurities. Adding a soluble impurity generally decreases the surface tension ($\sigma_{LG}$) of the liquid.

Step 3: Determine the change in contact angle. A decrease in $\sigma_{LG}$ typically results in a smaller contact angle ($\theta$). A smaller contact angle indicates better wetting, meaning the liquid spreads more easily across the solid surface. $$ \therefore \text{The angle of contact decreases when a soluble impurity is added.} $$