Intermolecular force:

The force of attraction or repulsion exists between the molecules and is called intermolecular force. This is a short range force and does not obey inverse square law. Intermolecular force is of two types: cohesive force and adhesive force.

Cohesive force:

The force of attraction between two molecules of the same substance is called cohesive force.

Cohesion:

The force of attraction between two molecules of the same substance is called cohesive force. This property is called cohesion. The definite shape and rigidity of a solid body is due to cohesion. It is strongest in solids.

Adhesive force:

The force of attraction between two molecules of the different substance is called adhesive force. Force of attraction between water and solid is adhesive force.

Range of molecular force:

The maximum distance between two molecules up to which intermolecular forces are effective is called range of molecular attraction. In solids and liquids it is nearly equal to 10^–9 m and hence it is called short range force.

Sphere of influence:

An imaginary sphere drawn, with given molecule as centre and radius equal to the molecular range is called the sphere of influence.

Surface film:

The layer of surface of liquid whose thickness is equal to range of molecular attraction is called surface film.

Surface energy:

The potential energy per unit area of the liquid surface under isothermal condition is called surface energy per unit area. SIU of surface energy is joule or J.

Surface tension:

The force per unit length acting at right angles to an imaginary line drawn on the free surface of liquid is called surface tension. SIU of surface tension is N/m.

Applications of surface tension:

(a) soap water or detergent water is used for cleaning clothes because soap or detergent reduces the surface tesion of water, consequently water penetrates more through pores of the clothes,

(b) tooth paste moves more freely in the mouth because it contains soap which reduces the surface tension,

(c) mosquito eggs float on water because of surface tension. If kerosene is added to water, thin film of kerosene floats on water, then surface tension decreases and mosquito eggs sink in water and destroyed,

(d) petrol is sprayed on sea water before storm, petrol reduces the surface tension of water and consquently height of water waves is reduced.

Relation between surface energy and surface tension:

The surface tension is numerically equal to surface energy per unit area.

Angle of contact:

When a liquid is in contact with a solid, the angle between tangent drawn to the free surface of the liquid and the surface of solid at the point of contact measured inside the liquid is called angle of contact.

Characteristics or properties of angle of contact:

(a) for a given solid-liquid pair, the angle of contact is constant,

(b) the value of angle of contact depends upon nature of liquid and solid in contact,

(c) it also depends upon the medium which exists above the free surface of liquid,

(d) the angle of contact changes due to impurity,

(e) the angle of contact changes due to temperature.

Shape of liquid drops and bubbles:

In sphere, suface area is minimum for a given volume. The shape of liquid droplets and bubbles floating in air is spherical because of surface tension. Surface tension tries to minimize the surface area of droplet and bubble resulting in spherical shape.

Rain-coats:

The threads of rain-coat are coated with water proofing agents like resin which have very small force of adhesion with water so rain-coats become water proof.

Capillarity:

The phenomenon of rise or fall of a liquid inside a capillary tube when it is dipped in the liquid is called capillarity.

Applications of capillarity:

(a) sap and water rises even up to the topmost leaves in the tree by capillary action,

(b) water rises up the creaves in rocks due to capillary action,

(c) cloth rag soaks water due to capillary action,

(d) blotting paper absorbs ink due to capillary action,

(e) in fountain pen, ink flows through nib because of capillary action,

(f) rockel or spirit rises up the wick of lamp because of capillary action.

Rise of liquid in a capillary tube:

Height ‘h’ of a liquid column in a capillary tube is given by the following expression:

h = 2 T cosθ / (r ρ g)

This formula can also be used to compute surface tension ‘T’ of liquid as follows:

T = h r ρ g / (2 cosθ)

where, h = height of liquid column in a capillary tube, T = surface tension of liquid, θ = angle of contact of liquid with capillary tube, r = radius of capillary tube, ρ = density of liquid, g = gravitational acceleration due to gravity = 9.81 m/s².

Effect of impurities on surface tension:

(a) When highly soluble impurity like common salt is dissolved in water, surface tension of water increases. When a sparingly soluble substance like phenol or alcohol is dissolved in water, surface tension of water decreases, (b) If insoluble impurity added into water, surface tension of water decreases.

Critical temperature:

The temperature at which surface tension of liquid becomes zero is called the critical temperature of the liquid.

Effect of temperature on surface tension:

The surface tension of almost all liquids decrease with rise in temperature. If T and T₀ are the values of surface tension of liquid at θ^oC and 0^oC respectively, then over a certain range of temperature, we have the relation:

T = T₀ (1 – α θ)

where α is constant which depends on the nature of liquid.

Effect of contamination on surface tension:

The presence of dust particles or lubricating materials on the liquid surface decreases its surface tension.