Question

When the angle of contact between a solid and a liquid is ${90^0}$ , then
A. Cohesive force $ > $ Adhesive force
B. Cohesive force $ < $ Adhesive force
C. Cohesive force $ = $ Adhesive force
D. Cohesive force $ > > $ Adhesive force

Answer 1

Hint:
Angle of contact can be resolved using the concept of adhesiveness and cohesiveness for a liquid and solid wall, in which the liquid is kept. The angle of contact depends upon the composition of the ions, which is present in the liquid and the magnitude of cohesive and adhesive force.

Complete step-by-step solution:The angle of contact is determined as calculated value of angle, which is observed to form within the meniscus of the liquid and walls of columns of solid containing the liquid. In other words angle of contact between the tangent to liquid surface and surface of solid.
Cohesive force is the force between the same molecules and adhesive force is the force between different molecules. When the angle of contact is between the solid and liquid is ${90^0}$, cohesive force is equal to adhesive force because they will be in equilibrium state and we will get the flat meniscus in this case.
Hence, option (C) is correct.

Additional Information:
When adhesive force is greater than cohesive force, it helps in capillary rise and when cohesive force is greater than adhesive force , it falls the capillary level.

Note:- Study the other cases of contact of angle, which are as follows:
Case $1.$ When the angle of contact between the solid and liquid is less than ${90^0}$ , then cohesive force is less than adhesive force and observed concave meniscus.
Case$2.$ When the angle of contact between the solid and liquid is greater than ${90^0}$ , then cohesive force is greater than adhesive force and observed convex meniscus.