Question

Which of the following is not a colligative property?
A.Elevation in boiling point
B.Relative Lowering of vapour pressure
C.Osmotic pressure
D.Freezing point

Answer 1

Colligative properties are those properties which depend on the number of moles of solute present in the solution and not on the nature of solute. Dilute solutions containing non volatile solute show some specific properties which depend upon the relative number of solute particles and don't depend upon the nature of solute are called colligative property.

Complete step by step solution:
Colligative property is directly proportional to amount of solution or concentration. Colligative properties are of 4 types: Relative lowering of vapour pressure, Elevation in boiling point, Depression in freezing point and Osmotic pressure.
Relative lowering of vapour pressure: vapour pressure is the pressure applied by the vapour placed above the liquid on its liquid state in a closed vessel. It is a surface phenomenon. As it is the surface particles of the liquid which goes up and becomes vapour. But due to the non volatile solute, the surface area decreases and hence causes the relative lowering of vapour pressure compared to vapour pressure of pure solvent.
Elevation in boiling point: the temperature at which vapour pressure of liquid becomes equal to atmospheric pressure is called boiling point. When non volatile solute is added to the liquid, vapour pressure of solution decreases. More is the amount of solute added more will be a rise in vapour pressure and more heat will have to be provided to boil.
Depression in freezing point: the temperature at which vapour pressure of liquid becomes equal to vapour pressure of solid is called freezing point of liquid. As vapour pressure of solution containing non volatile solute is less than vapour pressure of liquid. Now to freeze this solution, its vapour pressure must be decreased by decreasing the temperature below the freezing point of liquid to make vapour pressure of solution equal to vapour pressure of solid.
Osmotic pressure: the minimum external pressure applied on the solution side which just stops the process of osmosis (movement of solvent molecules) is called osmotic pressure. As more the solvent molecules are there, more will be osmotic pressure.

Thus, the correct option is D.

Note: If a pressure greater than osmotic pressure is exerted on the solution side, then the flow of solvent molecules occurs from solution to solvent through semi permeable membranes, this is known as reverse osmosis or commonly abbreviated as RO. This is used for the purification of water.