Question

Explain why the angle of contact of mercury with glass is obtuse, while that of water with glass is acute.

Answer 1

Forces in action

1. Cohesive force: It may be defined as that force of attraction that is present between molecules of the same substance.
2. Adhesive force: It may be defined as that force of attraction that is present between molecules of the different substances.

Angle of contact

1. The cohesive force between water molecules is less than the adhesive force between the molecules of glass and water.
2. This results in the formation of a concave or acute meniscus on the surface of the water.
3. The adhesive force between the molecules of mercury and those of glass is less than the cohesive force between the molecules of mercury.
4. This results in the formation of a convex or obtuse meniscus for the surface of mercury.

Diagrammatic representation

Angle of contact is directly proportional to Cohesive and Adhesive forces.