Question

How to answer this question in board to gain cent marks.

Explain why the free surface of some liquid in contact with a solid is not horizontal.

Answer 1

The free surface of some liquid in contact with a solid is not horizontal. The reason for this is that the force of cohesion and adhesion are not balanced.

When the force of cohesion between two liquid molecules is less than force of adhesion between the liquid and the solid, the liquid surface curves up towards the solid. If such a liquid is poured into a solid tube, it will have a concave meniscus. For example, a glass rod dipped in water or water inside a glass tube.

On the other hand, when the force of cohesion is greater than the force of adhesion, the liquid surfaces get curved downward in contact with a solid. When such liquids are put into a solid tube, a convex meniscus is obtained. For example, a glass rod dipped in mercury or mercury within a solid glass tube.