Question

what is surface Tension ?

Answer 1

Surface tension is the force per unit length of the line of contact (at the boundary) between a liquid and a solid surface.  For example, when water is put in a narrow test tube, the surface of water touches the glass at the boundary of its surface.  At this boundary, there is a force upwards acting on the surface of water.  This is due to adhesive forces of glass on water molecules.

Answer 2

The liquid molecules which are below the surface are in equilibrium.  The liquid molecules which are at the surface are attracted by other molecules in downward direction. So, a force is originated to balance the surface molecules which is equal but opposite in direction to the downward force. This force is called surface tension.