All the liquids have a tendency to form the vapours. These vapours exert pressure on the liquid surface. “The pressure exerted by the vapours of a liquid over the liquid surface in equilibrium with the liquid at that temperature is called the vapour pressure.” Vapour pressure is the characteristic property of every liquid. The substances which easily form the vapours are called the volatile substances e.g. gasoline, ether, camphor, iodine etc. while the substance which do not easily form the vapours are called non-volatile substance. e.g. water, sugar etc.
Now consider that a small amount of solute is added to a pure solvent. in the solution some of the sites are occupied by solute and some by solvents at the surface of solution. Since the vaporization occurs from the surface of the liquid the vaporization of every compound is decreased. Thus the vapour pressure of a component is decreased when dissolved in another component (in comparison to its pure state). This is known as “lowering of vapour pressure”.
The exact quantitative relationship between the lowering of vapour pressure and amount of the component was put forward by Raoult in the form of Raoult’s Law. Discussed further.
Raoult’s Law:- according to Raoult’s Law:- “The vapour pressure of a component in solution at a given temperature is equal to the mole fraction of that component in solution multiplied by the vapour pressure of that component in pure state”.
For illustration let the two components in a binary solution are A and B with mole fractions xA and xB and the vapor pressure P0A and P0B in their pure satate. The V.P. of the components in solution (pA and pB) is given by Raoult’s law.
pA = xA X p0A
and pB = xB X p0B
thus the V.P. of the solution (pS) is given by:
pS = pA + pB
pS = xA X p0A + xB Xp0B --------------------(1)
Graphically it may be represented as:
Special case:- if the solute (A) is non Volatile substance, its vapour pressure (P0A) will be zero.
Substituting P0A = 0 in equation (1)
pS = xB X P0B ------------------(2)
Thus the vapour pressure of the solution containing non-volatile solute is due to the solvent only and is equal to the product of mole fraction of solvent and its vapor pressure in pure state.