Question

Mercury does not wet glass, wood or iron because:
(A) Cohesive force is less than adhesive force
(B) Cohesive force is greater than adhesive force
(C) Angle of contact is less than ${90^ \circ }$
(S) Cohesive force is equal to adhesive force

Answer 1

Hint To solve the question, you first need to know what cohesive and adhesive forces are and what they mean and how they affect the behavior of a liquid and its interaction with solid materials. Also, you need to know the properties of Mercury and the fact that even though it is liquid at room temperature, it is a metal.

Complete step by step answer
As mentioned in the hint section of the solution to the question, to solve the question, you foremost need to know what cohesive force and adhesive force is and how they affect the behavior of a liquid and its interaction with solid materials. Since Mercury is a liquid metal at room temperature, it also has cohesive and adhesive properties in it and acts just like other liquids in the case. Let us first explain what cohesive force and adhesive forces are:
Cohesive forces are collective intermolecular forces responsible for the bulk property of liquids resisting separation among the molecules of the liquid. These attractive forces, called cohesive forces, exist between molecules of the same substance and are responsible for maintaining the structure of the liquid drops or droplets by pulling the molecules together. These forces are the reason why a drop of water takes a circular shape, so as to keep the molecules as close as possible and decrease the surface area of the water droplet.
On the other hand, the term adhesive forces refers to the attractive forces that exist between two or more substances which are unlike each other. There can be various forces contributing to it like mechanical force, electrostatic force, magnetic force, etc. The higher the adhesive forces, the more likely a liquid is supposed to make the surface of contact wet.
Since adhesive forces are responsible for making the surface wet by pulling the particles out of their usual formation created due to cohesive forces, if the adhesive forces are weaker than the cohesive forces of the liquid, the liquid would not make the surface of contact wet.
As it is told in the question that mercury does not wet glass, wood or iron, it is so because the cohesive forces are greater than the adhesive forces for the respective substances.

Hence, the correct answer is option (B).

Note A common mistake that is to be seen that many students take adhesive forces as the forces responsible for the adhesion of the molecules of the liquid, and thus, reverse the roles of both of the forces and thus mark the exact opposite option as the right one, thus, lose marks.