What are colligative properties
As we all are know that when we add non volatile solute in volatile solvent then it’s vapour pressure decreases. There are many properties of solutions which are connected with this decrease vapour pressure like relative lowering of vapour pressure of the solvent, depression of freezing point of the solvent, elevation of boling point of the solvent and one last is osmotic pressure of the solution. All these properties depends on the number of solute particles irrespective of their nature relative to the total number of particles present in the solution. Such kinds od properties are called colligative properties. So we can say that the properties of solution which depend only on the number of solute particles.
Four colligative properties are
1. Relative Lowering of Vapour Pressure
When non volatile solute is dissolved in solvent vapour pressure of the solution becomes lower than that of pure solvent which is known as Relative Lowering of Vapour Pressure.
2. Elevation of Boiling Point
As we know that the boiling point is a temperature at which its vapour pressure becomes equal to the atmospheric pressure. The Boiling point of solution is always higher than boiling point of pure solvent in which solution is preapared. This is because vapour pressure of solution at a given temperature is found to be lower than that of vapour pressure of pure solvent at the same temperature.
3. Depression of freezing point
The freezing point of a solution is the temperature at which vapour pressure of the substance in its liquid phase is equal to its vapour pressure of solid phase. When non volatile solute is added to a solvent the freezing point of the solution is always lower than that of pure solvent as vapour pressure of solvent decreased in presence of non-volatile solute.
4. Osmotic and Osmosis Pressure
The process of flow of solvent molecules from solution of lower concentration to solution of higher concentration through semipermable mambrane is known as osmosis.The hydrostatic pressure which develops on account of osmosis is called osmotic pressure. The pressure which just stops the flow of solvent is called osmotic pressure of solution.
Reverse osmosis
If a pressure larger than the osmotic pressure is applied to solution side than the pure solvent flows out of the solution through semipermable membrane this phenomena is called Revese Osmosis.
Revese osmosis is used in distillation of sea water.
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