Question

Soap helps is cleaning clothes, because
A. It attracts the dirt particles
B. It decreases the surface tension of water
C. It increases the cohesive force between water molecules
D. It increases the angle of contact

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 answer the given question.

Complete answer:
Let us assume that we inserted a glass tube in water. The contact angle of pure water and glass tube will be zero degrees. But adhesive forces will dominate. So initially the surface of water will be concave. Then to reduce surface area, the resultant contact force will pull the water surface up and then the surface will become flat. The surface tension is the reason for this. Same cohesive and adhesive force phenomenon will be the reason for this too.
If we repeat the same thing with mercury then the result would be exactly opposite, that means the mercury level would fall down.
These are because the contact angle between mercury and glass is 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. Hence it is generally told that liquid can only wet the surface if adhesive force dominates over cohesive force, or else it can’t wet just like mercury.
When we add soap to water, it will reduce the cohesive forces between water molecules and so that surface tension also decreases. So water will wet the cloth easily and dirt can be removed.

Hence, option B is the correct answer.

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. Hence option D cannot be answered for the given question. Contact angle decreases here.