Explanation:

If we consider a molecule such as “A” which is inside the liquid. This molecule is attracted by cohesive force from all directions by neighboring molecules of the liquid.

The forces which act on the molecule such as “B” which is near the surface of the liquid are:

1-  Adhesive forces act upward between the molecules of the  liquid & the molecules of the air but these forces are so small that we neglect them.

2- Cohesive forces act horizontally between the molecule of the liquid “B” & the other molecules of the liquid. These forces are horizontal & equal in all directions so the resultant forces is equal Zero.

SO the molecule “B” is attracted only by a force due to molecules of the liquid downwards forming a semi-sphere with the molecule “B” as its center, Thus the resultant of such forces will pull the molecule “B” downwards.

What happens to the molecule “B” can occur to all molecules on the surface of the liquid, so these molecules lie under the effect of the vertical forces act on the surface of the liquid & try to attract them to inside the liquid.

Therefore the number of molecules at the surface of the liquid decreases.

A hole is produced due to the decrease of the molecules of the liquid on the surface & attraction forces (cohesive forces) between the molecules make the molecules come close together, so the area of the surface of the liquid decreases.

Surface Tension

·        Place a thin steel needle covered with a thin greasy gently on a water surface.
Put a thin steel needle over a filter paper, then the paper is placed gently on a water surface, after a while the filter paper will sink & the needle will float.

·        The thin surface layer of water is distorted under the weight of the needle this means that the needle does not float at the level of the surface of the water, (because density of iron is greater than density of water) but water membrane carries it.

·        The surface of the water acts as an elastic water membrane & prevents the needle from sinking. This property is called Surface Tension.

Expressions about the surface tension:

·        If a small iron is placed carefully on water surface it makes a small depression in the surface & rests there without sinking.

·        The floating of a steel needle on the water surface.

·        Spherical shape of the mercury drops (the smallest area of a given volume).

·        Small insects can walk on the surface of water.

·        The rise of the liquid in the capillary tubes.