Important Topic of Chemistry: Colligative Properties

What Are Colligative Properties?

Colligative qualities are those of a solution that is determined by the number of particles in a given volume of solvent (concentration), rather than the mass or identity of the solute particles. The temperature has an impact on colligative characteristics as well. Colligative Properties.

Colligative properties are physical properties of a solution that depend on the number of solute particles dissolved in a solvent, not on the chemical nature of the solute. Examples of colligative properties include boiling point elevation, freezing point depression, osmotic pressure, and vapor pressure lowering.

These are 4 Colligative Properties

1. Relative lowering of vapour pressure: A liquid’s vapour pressure is the pressure of the vapour that is in equilibrium with it. When a non-volatile solute is dissolved in a solvent to produce a solution, the vapour pressure of the solvent is reduced. Raoult’s law gives the equilibrium vapour pressure for an ideal solution.
2. Boiling point elevation: The addition of a solute to a solution stabilizes the solvent in the liquid phase and decreases the solvent chemical potential, reducing the tendency of solvent molecules to migrate to the gas or solid phases. As a result, liquid solutions slightly above the solvent boiling point become stable at a given pressure, increasing the boiling point. In systems where the solute is primarily limited to the liquid phase, these features are complementary. For non-volatile solutes where the solute present in the gas phase is insignificant, boiling point elevation (like vapour pressure reduction) is complementary. Because very few solutes dissolve significantly in solid solvents, freezing point depression is colligative for most solutes. The boiling point is the temperature at which the liquid and gas phases are in balance. The amount of gas molecules condensing to liquid equals the number of liquid molecules evaporating to gas at the boiling point. Adding a solute lowers the rate of evaporation by diluting the concentration of liquid molecules. The boiling point happens at a higher temperature to compensate for this and re-establish equilibrium.
3. Depression at freezing point (cryoscopy): The addition of a solute that is insoluble in the solid solvent lowers the freezing point of a pure solvent, and cryoscopy is the measurement of this difference. Liquid solutions that are just below the freezing point of the solvent become stable, lowering the freezing point. In a dilute solution, the increase in boiling point and decrease in freezing point is proportional to the decrease in vapour pressure.
4. Osmotic pressure: The difference in pressure between the solution and the pure liquid solvent when the two are in equilibrium across a semipermeable membrane that permits solvent molecules but not solute particles to pass is called the osmotic pressure of a solution. Osmosis is the net movement of solvent over the membrane into the solution when the two phases are at the same starting pressure.

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Different Types of solutions

Isotonic solutions: Isotonic solutions are made up of two liquids with the same osmotic pressure and temperature. When such liquids are separated by a semipermeable membrane, there is no osmosis. 

• Hypotonic solution: A hypotonic solution has a lower osmotic pressure than the surrounding environment, meaning the concentration of solute particles is lower. When a semipermeable barrier separates the hypotonic solution from the water, water moves out of the hypotonic solution.
• Hypertonic solution: A hypertonic solution has a higher osmotic pressure than the surrounding environment, meaning the concentration of solute particles is higher. Water moves inside the hypertonic solution if the hypertonic solution is separated by a semipermeable membrane.