Question

Assertion: Meniscus is formed in case of confined liquids.
Reason: It is not due to the force of surface tension.
A. Both assertion and reason are correct and reason is the correct explanation of assertion.
B. Both assertion and reason are correct but reason is not the correct explanation of assertion.
C. Assertion is correct but reason is incorrect.
D. Both assertion and reason are correct.

Answer 1

Hint:The raising or lowering of water surface in a tube or capillary can only take place due to surface tension. Surface tension is the result of different forces that act on the molecule present on the surface of water. These forces are cohesive force, adhesive force and gravitational force.

Complete answer:
Meniscus is defined as the surface tension of the liquid when it is confined in a tube or a capillary. The surface of liquid is subjected to surface tension. Any molecule on the surface of liquid is under the influence of three forces. They are;

1. Cohesive Force: It is due to the force of attraction between molecules of the liquid. This force acts inwards at an angle of \[{45^ \circ }\]with the vertical. This force forms a concave meniscus as the molecules are strongly attracted to each other. This can occur with water and a glass tube.

2. Adhesive force: It is due to attraction between the forces of the container (tube or a capillary) and the liquid molecule. It acts at right angle to the wall of the tube. This force forms a convex meniscus as the molecules are strongly attracted to the walls of the container holding the liquid. This can occur with mercury and a glass.

3.Gravitational force: The gravitational force on the molecule acts vertically downwards.Thus, the meniscus is formed in case of confined liquids due to the effect of surface tension. Therefore, the assertion is correct but the reason is wrong.

Hence, option C is the correct answer.

Note: The gravitational force of one liquid molecule is so small that it can be neglected (only in the case of one liquid molecule). The cohesive and adhesive forces are intermolecular forces. Both these forces are short range forces and exist only between the nearest neighboring molecules. They decrease with distance more rapidly than inverse square law forces.