The intermolecular forces acting at all sides of a molecule located inside the bulk of liquid will cancel out and a molecule inside the bulk does not experience any net force. But a molecule, located on the surface of the liquid, experiences net attractive force directed towards the interior of the liquid (Figure 6.12), due to the molecules below it and as there are no molecules above it. This net force is the cause of surface tension. The SI unit of surface tension is N/m (newton per meter).

Surface tension is defined as a cohesive effect of the surface of the liquid due to the forces between atoms or molecules of liquids. This phenomenon is mostly observed in the formation of soap bubbles and when a razor blade is placed on water surface with its flat part.

The magnitude of surface tension of a liquid depends on the attractive intermolecular forces between the molecules. When this force is large, the surface tension is large. Surface tension can be reduced by heating the liquid so that dispersive thermal energy is increased and the effectiveness of intermolecular attraction is decreased. Some examples of surface tension are given below.

• If water is poured using a water dropper, the water falls in a series of drops but does not flow in a continuous stream. The shape of the drops is formed by the surface tension of the water. The falling drops of water are not perfectly spherical as the force of gravity distorts them. What would be the shape of a drop of water in the absence of gravity? Why? The drop would minimize the surface area in order to minimize tension, which would result in a perfectly spherical shape.
• Several insects, such as the water strider, are able to walk on water. They form their legs such that it distributes its weight causing a depression on the water surface. This minimizes the potential energy to create a balance of forces so that the strider can move across the surface of the water without breaking through the surface.
• Although the density of needle is greater than that of water, it is common experience to see it floating. This is because; the surface tension along the depression is enough to balance the force of gravity pulling down on the steel needle.