what is molecular theory of surface tension?

Hi!

 

Surface tension is the stretching of the liquid surface like a membrane which tends to contract so as to occupy a minimum surface area in a given situation. Have a look at the given figure for a better understanding of molecular theory of surface tension.

There is a intermolecular force of attraction in any liquid which is called cohesive force. For the molecules below surface, this force is balanced, because it is surrounded by the similar molecules. But, for a molecule at the surface, these forces are not balanced as there are no similar molecule  above the surface. Hence there is a net downward pull on this molecule, because of which it behaves like a stretched membrane. This phenomenon is called surface tension.

 

Hope it is clear to you!

Best wishes!!