Surface tension tends to minimize the surface area of the liquid. Surface tension can be defined as the tangential force per unit length acting perpendicularly to the line drawn in the surface of the liquid. Its unit is Nm^-1 in SI system and dyne/cm^-1 in CGS system.

FACTORS AFFECTING SURFACE TENSION

1. Intermolecular force of attraction: The liquid which has greater strength of an intermolecular force of attraction will have greater surface tension. The greater the intermolecular force of attraction, the surface molecules of the liquid experience greater resultant pull.

2. Temperature: When temperature is increased, the liquid molecule possesses greater kinetic energy which overcomes the intermolecular force of attraction. So the surface tension decrease with an increase in the intermolecular force of attraction.

3. Presence of impurities: The surface-active impurities decrease the surface tension of the liquid while bulk active impurities increase the surface tension of the liquid. The impurities which concentrate on the surface of the liquid are called surface-active impurities. For eg: soap and detergent. The impurities which concentrate the bulk of liquid are called bulk active impurities. Eg: NaCl, KCl.

1. i. Cohesive force: The intermolecular force of attraction between the same kinds of molecules is called cohesive force. For eg: the force of attraction between water molecules or inert-atomic force of attraction among mercury atoms.
2. ii. Adhesive force: The force of attraction between different types of molecules is called adhesive force. For eg: force of attraction between water molecules and glass molecules.