Surface Tension (γ)

Ø The force acting per unit length or energy per unit area of the surface of a liquid is called the surface tension of that liquid.

Explanation: A molecule in the bulk of liquid experiences equal intermolecular forces from all sides. The molecule, therefore, does not experience any net force. But for the molecule on the surface, the net attractive force is towards the interior of the liquid (See figure), due to the molecules below it. Since there are no molecules above it. These downward attractive forces are responsible for the surface tension.

Ø The surface tension tends to reduce the surface area of the liquid.

Ø Freely falling drop of liquid is spherical due to surface tension as sphere has the smallest ratio of surface area to volume.

Unit of Surface Tension:

The measuring unit of surface tension is dynes/cm or ergs/cm² (1 dynes/cm = 1 erg/cm³)

Surface tension (dynes/cm or ergs/cm²) at 20 °C

Liquid	Surface tension (γ)
Water	72.583
Ethyl alcohol	22.03
Chloroform	27.1
Acetone	23.7
Benzene	28.88
Mercury	471.6

Ø Capillary action of liquid (liquid rises in the capillary tube) is also due to their surface tension.

Ø   Like viscosity, surface tension also decreases with increasing of temperature.