COLLIGATIVE PROPERTIES OF LIQUID

Colligative properties are those properties of a liquid which depend on the number of solute particles and not on the concentration of the solution. These properties are studied in liquids. As per these properties, the mixing of a non-volatile solution in a volatile solution shows a decrease in the relative vapour pressure of the solution.
Furthermore, this decrease in the vapour pressure can be further used to quantify and study the properties of all liquid solutions. Specifically, colligative properties are dependent upon the solute particles present in a given solution. Derived from the Latin word ‘Coligare’, the word, colligative refers to ‘binding together’.
There are four colligative properties namely- relative lowering of vapour pressure, elevation of boiling point, depression of freezing point and osmosis and osmotic pressures. Related to these properties, the following deductions can be made:
Relative Lowering of Vapour Pressure 
When a non-volatile solute is added to a solvent, the vapour pressure gets lower. This phenomenon is the lowering of vapour pressure. A relation between the pressure of the solution, the vapour pressure of the pure solvent and the mole fraction of the solute was discovered by a French Chemist.
It was also observed by him that it is mainly the concentration of the solute particles which was responsible for the lowering of vapour pressure. As per the laws devised by scientists at that time, Decrement in Vapour Pressure = Vapour Pressure of Pure Solvent – Vapour Pressure of Solvent. It is through this equation that we can determine the ultimate molar mass of a solute.
(Psolvent = Xsolvent Posolvent)

2) Elevation of Boiling Point

The vapour pressure of a solute tends to decrease as a non-volatile solute is added in a solvent. What is to be noted here is that the boiling point of such solution is always greater than the pure solvent in which it is added. This happens because the pressure of vapour is in direct proportion to the temperature of the solution.
If the solution is to be boiled, the temperature of such a solution has to be raised. This phenomenon comes to be identified as the elevation of boiling point. The solute particles in the solvent, along with the vapour pressure, together play a role in this phenomenon.
Formula: ΔT = iKbm
where ΔT= change in temperature
i = the van’t Hoff factor, which is the number of particles into which the solute dissociates
m = the molality, which is the moles of solute per kilograms of solvent
Kb = the molal boiling point constant (for water, Kb = 0.5121oC/m)

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