Explain surface tension on the basis of molecular theory.

Explanation

Let us consider three different molecules A, B and C in a given liquid a shown in Figure. Let a molecule 'A' be considered well inside the liquid within the sphere of influence. Since this molecule interacts with all other molecules in all directions, the net force experienced by A is zero. Now consider a molecule 'B' in which three-fourth lies below the liquid surface and one-fourth on the air. Since B has more molecules towards its lower side than the upper side, it experiences a net force in the downward direction. In a similar way, if another molecule 'C' is selected on the liquid surface (i.e., upper half in air and lower half in liquid), it experiences a maximum downward force due to the availability of more number of liquid molecules on the lower part. Hence it is obvious that all molecules of the liquid that falls within the molecules range inside the liquid interact with the molecule and hence experience a downward force.

When any molecule is brought towards the surface from the interior of the liquid, work is done against the cohesive force among the molecules of the surface. This work is stored as potential energy in molecules. Hence the molecules on the surface will have greater potential energy than that of molecules in the interior of the liquid. But for a system to be under stable equilibrium, its potential energy (or surface energy) must be a minimum. Hence, in order to maintain stable equilibrium, a liquid always tends to have a minimum number of molecules. In other words, the liquids tends to occupy a minimum surface area. This behaviour of the liquid gives rise to surface tension.