In the interior of a liquid, each molecule is attracted equally by the molecules around it. Yet at the surface, the molecules are subjected to cohesive forces only from below and from the side, the molecules in the air above the liquid having little or no effect. The net result is that the molecules at the surface are pulled in ward and closer together-leaving the surface in a state of constant tension and making the liquid behave as through it were covered by invisible elastic membrane.

One needs two only watch a dripping faucet to observe the peculiar way in which water first forms a horizontal film that slowly bulges as more water collects. Even as the weight of the water increases, the film does not break, but seems to tear itself away from the rim of the faucet and form a drop. This tendency of water, water or any liquid, to stick to itself and pull itself together is known as surface tension and is a direct consequence of the attraction of molecules of a liquid for one another.

As strong as this mutual attraction may be, sometimes an even stronger attraction for outside molecules exists. This is the case when water meets clean glass. Molecules of water will cling to glass, spreading over it in a sheet and wetting it thoroughly. If the glass is spotted by oil or grease, however, surface will triumph and the water will bead up instead.

(i)The tendency of a liquid to stick to itself and pull itself together is known as –