Surface Tension

Surface Tension - Liquid surface behaves like a stretched membrane. This is because liquid tries to occupy least surface area. This property of liquid is called surface tension (T).

Examples

1. Oil & water do not mix with each other.
2. Liquid drops spherical in shape.
3. Small insects can easily walk on the surface of water.
4. Needle can float on the surface of water though density of steel is more than water.
5. Cleaning of clothes by soaps & detergents; which lowers the surface tension of the water.
6. Clinical test for jaundice.

some definition from Surface Tension

Molecular force - The force which keeps the molecules together is called molecular force. 

Cohesive force - It is defined as the force of attraction between molecules of same substances. E.g. force of attraction bet two water molecules.

Adhesive force - It is defined as the force of attraction between molecules of different substances. E.g. force of attraction between water molecules & glass molecules.

Molecular Range - The maximum distance to which cohesive force can act is known as molecular range.

Sphere of influence - An imaginary sphere surrounding a molecule where force of attraction is present is called sphere of influence. The radius of sphere is its molecular range.

Note - Red circles represents the molecule of glass whereas black circles represents the molecule of liquid.

Angle of contact - It is the angle between the curved part of liquid surface at the point of contact with the wall of container. Angle of contact is always constant for given solid-liquid pair.

Water	Mercury
Water level creeps up at the wall	Mercury level depressed down at the wall.
Force of adhesion is stronger than cohesion.	Force of cohesion is stronger than adhesion.
Water wets glass.	Mercury does not wet the glass.
Angle of contact is acute (<90°).	Angle of contact is obtuse (>90°).
Shape of liquid surface is concave.	Shape of liquid surface is convex.