Question

Which one of the following statements about the angle of contact $(\theta)$ is wrong?

• A. $\theta$ is not constant for particular solid–liquid pair.
• B. $\theta>0^\circ$ for pure water–glass pair.
• C. $\theta>90^\circ$ for mercury–glass pair.
• D. $\theta<90^\circ$ for kerosene–glass pair.

Hint:

Angle of contact depends on the nature of solid–liquid pair and surrounding medium, not on the amount of liquid.

Answer 1

The Correct Option is A

Step 1: Define angle of contact.
Angle of contact is the angle between the tangent to the liquid surface at the point of contact and the solid surface, measured inside the liquid.
Step 2: Nature of angle of contact.
For a given solid–liquid pair and given surrounding medium, the angle of contact remains constant.
Step 3: Check each option.
- Pure water wets glass, so $\theta$ is small but greater than $0^\circ$.
- Mercury does not wet glass, hence $\theta>90^\circ$.
- Kerosene wets glass, so $\theta<90^\circ$.
Step 4: Identify the incorrect statement.
Statement (A) is wrong because angle of contact is constant for a particular solid–liquid pair.