The properties of dilute solutions depend only on number of particles of solute (molecules or ions) present in the solution and not on their nature, are called colligative properties.
The important colligative properties are
1. Relative lowering of vapour pressure
2. Elevation of boiling point
3. Depression of freezing point
4. Osmotic pressure
1. Relative lowering of vapour pressure
2. Elevation of boiling point
3. Depression of freezing point
4. Osmotic pressure
Note: Colligative properties are the properties of only dilute solutions which are supposed to behave as ideal solutions.
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1. Relative lowering of vapour pressure:
According to Raoult's law,
Vapour pressure of solvent in the solution = mole fraction of solvent in the solution x vapour pressure of pure solvent
| PA = P°AXA | P°A = V.P. of pure solvent |
| PA = P°A(1 - XB) | XA + XB = 1 |
| PA = P°A - P°AXB |
When solute is non-volatile,
Then, V.P. of solvent in the solution = V.P. of solution = P
Then, V.P. of solvent in the solution = V.P. of solution = P
P = P°A - P°AXB
| P°A - P = P°AXB | P A - P = lowering in vapour pressure | ||
|  = Relative lowering of vapour pressure | ||
For dilute solution nA + nB  nA | XB =  | ||
 | nA =  | ||
nB =  |
= XB |
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