Explain Surface Tension and Surface energy.

Hint: To answer this question, we need to compare the potential energy of a liquid molecule inside the bulk of the liquid with a molecule at the free surface. The extra potential energy possessed by the free surface of the liquid will be the surface energy, and the corresponding force exerted by the surface will be the surface tension.

Complete Answer:
We know that the liquids, unlike the gases, have a definite volume. They don’t have any definite shape but they take the shape of the container in which they are placed. Moreover, when placed inside a container, they acquire a free surface. The molecules of the liquid on this surface possess some additional energy, as compared to the molecules well inside the bulk of the liquid. This additional energy is known as the Surface energy.
A molecule inside the bulk of the liquid is surrounded by the other molecules from all of the sides. This molecule is attracted from all the sides by the other molecules, due to which it possesses negative energy. But a molecule at the free surface is only partially surrounded by the other molecules. So its potential energy is less negative. Thus, the free surface of a liquid has more energy than the other molecules in the interior. Therefore, a liquid tends to have the least surface area.
For this reason a liquid tends to contract or shrink to the minimum surface area. So the free surface of a liquid behaves like a stretched sheet. So it opposes the external force which is applied on it, through the force of Surface tension.
Mathematically, force exerted by the free surface of a liquid per unit length is known as the surface tension. It is also equal to the surface energy of a liquid per unit area.

Note:
The Surface tension is responsible for many interesting phenomena occurring in nature. For example, some insects are able to walk on the surface of water due to the reaction force offered by the surface tension of water. Also we know that although a needle is denser than water, it still floats on the surface of water. This is also due to the surface tension.