Question

When the angle of contact between a solid and a liquid is ${90}^{\circ }$, then

• A. Cohesive force>Adhesive force
• B. Cohesive force=Adhesive force
• C. Cohesive force<Adhesive force
• D. Cohesive force>>Adhesive force

Answer 1

The correct option is B

Cohesive force=Adhesive force

The angle of contact:

1. The angle between the meniscus and the walls of the column of liquid is called the angle of contact.
2. The angle of contact is generally measured from the vertical wall below the surface of the liquid to the tangent to the meniscus.

Consider the following diagram:

Cohesive force and Adhesive force:

1. Cohesive force is the force acting between the molecules of the same materials.
2. The adhesive force is the force, that acts between the molecules of the different materials.
3. When the angle of contact is ${90}^{\circ }$, the meniscus is found to be flat. Hence, the system is in equilibrium.
4. As the system is in equilibrium, the adhesive force, and the cohesive force will be equal in magnitude.

Hence, option (B) is correct.

Additional information:

1. When the angle of contact is less than ${90}^{\circ }$, the convex meniscus is observed. Hence, the cohesive force will be less than the adhesive force.
2. When the angle of contact is more than ${90}^{\circ }$, the concave meniscus is observed. Hence, the cohesive force will be greater than the adhesive force.