Question

When wax is rubbed on cloth, the cloth becomes water proof why?

Answer 1

The force of interaction between alike molecules is called a cohesive force while the force of interaction between two different molecules is called adhesive force. we will discuss how these forces can explain the reason for the given question.

Complete answer:
When we insert the tube in the vessel of water then due to capillary action even though initially the water surface would be concave and due to the domination of adhesive forces the water would go up in the capillary and the surface would become flat in order to reduce the surface area and to reduce the surface energy. The surface tension is the reason for this. Same cohesive and adhesive force phenomenon will be the reason for this too.
For mercury if we follow the same case the mercury level will fall down.
These are because the contact angle between mercury and glass nis very high while the contact angle between water and glass is low. Hence we call water will wet the surface and mercury will not wet the surface. That is when cohesive forces dominate the substance cannot wet the surface and when adhesive forces dominate the substance can wet the surface.
So generally water will wet the cloth because the adhesive force between water and cloth surface dominate than the cohesive force between the water. But when we rub the cloth with wax the cohesive force between water will dominate than the adhesive force between water and wax on the cloth surface. Hence water cannot wet the cloth rubbed with wax. So cloth becomes water proof.

Note:
In short we can conclude that if the contact angle is more then the liquid will not wet the surface just like mercury and if contact angle is less then liquid will wet the surface just like water.