Question

Before the phenomenon of surface tension was understood, physicists described the surface of the water as being covered with a “skin.” What causes this skin-like phenomenon?

Short answer

The “skin” term which was being used by a physicist for the surface of the water is due to the unbalanced forces of attraction which are present on the surface than in the bulk of water.

Step-by-step solution

Intermolecular forces of attraction

Every liquid has some intermolecular forces of attraction depending on the nature of the liquid. But it should be noted that these forces present in liquid are different in the bulk and surface.

Unbalanced forces on the surface

On the bulk-interior surface, the number and strength of interactions on all the sides are equal, thus the net force of attraction is zero.

Compared to this, the surface molecules experienced only the downward force of attraction and their corresponding upward force is missing. This makes the surface molecules comparatively less stable, and thus they get contracted to each other because of the net downward force of attraction.

Surface Tension

This unbalanced force that causes a tensional force on the surface molecules is known as surface tension. Herein, the surface molecules move towards each other under the net force thus reducing the surface area of the liquid. These outer surface molecules are termed “skin”.