Surface Tension

The properties of a surface are quite different from the properties of the bulk material. A molecule well inside a body is surrounded by similar particles from all sides. But a molecule on the surface has particles of one type on one side and of different type on the other side.

The attractive forces between the molecules of a substance are called cohesive forces. The attractive forces between the molecules of a different substances are called adhesive forces.

A molecule of water well inside the bulk experiences cohesive forces but a molecule at the surface experiences both cohesive and adhesive forces. This asymmetric force distribution is responsible for surface tension.

The maximum distance up to which the cohesive force between two molecules exists is called the molecular range and of the order 10^-9 m or 1 nm. An imaginary sphere drawn around a molecule with a radius of molecular range is called the sphere of influence of that molecule.

A molecule in the surface has greater potential energy than a molecule well inside the liquid. The extra energy that a surface layer has is called the surface energy. Surface tension of a liquid is also equal to the surface energy per unit surface area.

Factors influencing surface tension

Temperature: Usually it decreases with increase of temperature. When temperature of liquid increases K.E. of its molecules also increase. Hence cohesive forces between the molecules become weak. At critical temperature the interface between liquid and its vapour disappear and so surface tension of a liquid becomes zero.

Impurities: If the liquid surface is contaminated by impurities, its surface tension decreases. For example, when oil or kerosene is sprayed on water surface, its surface tension decreases.

Capillarity

The property of rise or depression of the liquid due to surface tension in a tube is known as capillarity.

Flow of ink through a nib is due to capillarity.