Question

If a glass rod is dipped in mercury and withdrawn out, the mercury does not wet the rod because:
(A) Angle of contact is acute
(B) Cohesion force is more
(C) Adhesion force is more
(D) Density of mercury is more

Answer 1

Hint
The molecules of mercury are in stronger cohesive force with each other than the adhesive force between mercury and glass molecules. The behaviour of a liquid with a solid when comes in contact with each other is dependent on the strength of cohesive and adhesive forces.

Complete step by step answer
The angle measured between the surface of the solid with the liquid surface when a solid and liquid comes in contact is called the angle of contact. The surface of the liquid becomes concave in shape, if the angle of contact is acute. Similarly, if the angle of contact is obtuse, then the shape on the liquid surface will be convex. The mercury-glass angle of contact is obtuse and therefore option A is wrong.
Cohesive force is the attractive force between the same type of molecules and at the surface, there will be cohesive force directed inwards. Mercury does not wet the glass rod because; the mercury molecules are attracted to each other more than the attractive force between glass molecules and mercury.
Adhesive force is also an attractive force, but between the liquid molecules and the solid molecules. The water spreads over the glass and wets it, because the adhesive force between glass molecules and water molecules is stronger than the cohesive force between water molecules.
It is true that the density of mercury is high. It is nearly 13.6 higher than the density of water. But that is not the reason for not wetting the glass rod when dipped.
The glass rod is not wet when dipped in mercury because; the cohesive force of mercury molecules is more than the adhesive force between glass and mercury molecules.
So the correct answer is option (B).

Note
The surface tension of a liquid is due to the unbalanced cohesive force between the liquid molecules. At the bulk, the molecules are attracted to each other at all the directions due to the cohesive force. But, at the surface, the molecules are attracted inwards to the similar molecules due to cohesive force. Since, the force is in a single direction, the surface molecules are more attracted towards the bulk. This is how the surface tension is developed at the surface of a liquid.