Surface tension:

Molecules of the liquid at the surface are in different situation than those in the interior of the liquid. Molecule lying inside the liquid (As shown in figure as 'A') is surrounded by other molecules and so attracted equally in all directions. Hence the net force of attraction acting on the molecule is zero. But a molecule lying at the surface (As shown in figure as 'C') is attracted downwards by the molecules lying in the bulk of the liquid. Thus a molecule lying at the surface experiences a net inward attraction. As a result of this inward pull on all molecules lying at the surface, the surface behaves as if it were under tension. This property of liquid is called surface tension. Unit of surface tension is Dyne per cm.