The cohesive force between water molecules is less than that of the adhesive force between the glass and water molecule.
Due to this a concave meniscus is formed on the surface of water.
Thus, water makes an acute angle from the glass surface.
Again the cohesive force between mercury molecules is more than that of the adhesive force between the glass and mercury molecule.
Due to this a convex meniscus on the surface of the mercury.
Thus, mercury makes an obtuse angle from the glass surface.
The angle formed between water molecule and the glass is acute.
Water molecules have weak force of attraction between them and strong force towards solid.
Thus, they spread out easily.
Whereas in mercury, it makes obtuse angle with the glass and have strong force of attraction between them and weak force towards solid.
Thus, they form drops.
Surface tension is the force acting per unit length at an interface between the liquid and any solid.
It depends on the nature of the solid and not the area of the surface.
Water with detergent when get dissolved in it has small angle of contact.
When the angle is small, there will be fast capillary rise of the detergent of the cloth.
The capillary rise is directly proportional to the cosine of the angle of contact formed.
If the angle is small then the cosine of it will be large and thus the rise of detergent water in the cloth will be fast.
A liquid acquires the minimum surface area.
The surface area of the sphere is minimum for a given volume.
Thus, there will be no external force and liquid drops take spherical shape.
Final Answer:
(a) The angle of contact of mercury with glass is obtuse, while that of water with glass is acute. (b) Water on a clean glass surface tends to spread out while mercury on the same surface tends to form drops.
(c) Surface tension of a liquid is independent of the area of the surface.
(d) Water with detergent dissolved in it should have small angles of contact.
(e) A drop of liquid under no external forces is always spherical in shape.
