Colligative Properties and Their Types

Definition

Colligative properties are physical properties of solutions that depend only on the number of solute particles relative to solvent particles in a solution, and not on their chemical identity or mass.

Types of Colligative Properties

Property	Description	Example
Relative lowering of vapor pressure	Addition of nonvolatile solute reduces solvent vapor pressure (described by Raoult's law).	Dissolving salt or sugar in water lowers its vapor pressure compared to pure water.
Boiling point elevation	A solution boils at a higher temperature than the pure solvent.	Adding NaCl to water raises its boiling point above 100°C.
Freezing point depression	A solution freezes at a lower temperature than the pure solvent.	Ethylene glycol in car antifreeze lowers the freezing point of radiator water.
Osmotic pressure	The minimum pressure required to stop net solvent flow across a semipermeable membrane.	Cellular fluid balance maintained across cell membranes in biological systems.

Everyday Examples

• Road de-icing: Salt lowers the freezing point of water, melting ice on winter roads.
• Cooking pasta: Adding salt to boiling water slightly raises the boiling point temperature.
• Antifreeze in engines: Ethylene glycol depresses freezing point in winter and elevates boiling point in summer.
• Biological balance: Osmotic pressure prevents red blood cells from swelling or shrinking (crenation/hemolysis).

Key Notes

• Colligative properties strictly hold true for ideal dilute solutions.
• For electrolytes that dissociate (e.g., NaCl → Na⁺ + Cl⁻), the total number of particles increases, requiring the van 't Hoff factor (i) correction.
• Measurement of colligative properties (especially osmotic pressure) is widely used to determine the molar mass of macromolecules and polymers.

In short: Colligative properties depend on particle count, not type. They explain why salt melts ice, antifreeze protects engines, and osmotic pressure sustains biological life.

Explanation of Different Types of Colligative Properties

Relative Lowering of Vapour Pressure

When a non-volatile solute is dissolved in a pure solvent, the vapour pressure of the solvent decreases because the presence of solute molecules reduces the number of solvent molecules that can escape into the vapor phase. Consequently, the vapor pressure of the solution becomes lower than that of the pure solvent at the same temperature.
The relative lowering of vapour pressure can be mathematically expressed as:
Relative Lowering of Vapour Pressure = (p^o − p)/p^o
where p^o is the vapour pressure of the pure solvent, and p is the vapour pressure of the solution, (p^o − p) is lowering of vapour pressure.
We can also say that the relative loweing of vapour pressure is the ratio of lowering of vapour pressure to the vapour pressure of the pure solvent.

Elevation in Boiling Point

We know that the vapour pressure of solution is always lower than that of the solvent. Therefore, the boiling point of solution is always higher than that of solvent. In other words, when a non-volatile solute is dissolved in the pure solvent, the boiling point increases or elevated. This phenomenon is known as elevation in boiling point.

T₂ > T₁
and T₂ − T₁ = ΔT_b (Elevation in Boiling Point).

The elevation in boiling point (ΔT_b) is proportional to the concentration of the solute in the solution. It can be calculated as-
ΔT_b = i × K_b × m
Where, i is the Van't Hoff factor, K_b is the ebullioscopic constant and m is the molality of the solute.

For dilute solution, elevation in boiling point can be calculated by using the given formula-

Where, W is the weight of the solvent, w is the weight of the solute and m is the mass of the solute.

Depression in Freezing Point

When a non-volatile solute is dissolved in the pure solvent, the freezing point decreases. This phenomenon is known as depression in freezing point. When a liquid is cooled by dropping the temperature, the vapour pressure decreases as the quantity of vapour molecules decreases on the surface of the liquid. Unless supercooling occurs, the temperature finally stops decreasing and at the same time a few crystals of crystalline solids are formed and the liquid starts to freeze. At this temperature, both liquid and solid forms have the same vapour pressure. This temperature is called freezing point of liquid.
T₂ < T₁
and T₂ − T₁ = ΔT (Depression in Freezing Point).

The depression in freezing point is proportional to the molality of the added solute. The depression in the freezing point of a solution can be described by the following formula.
ΔT_f = i × K_f × m
Where, ΔT_f is the freezing point depression, i is the Van't Hoff factor, K_f is the cryoscopic constant, and m is the molality.

For dilute solution, depression in freezing point can be calculated by using the given formula-

Where, W is the weight of the solvent, w is the weight of the solute and m is the mass of the solute.

Osmotic Pressure

When two solutions are separated by a semipermeable membrane, then there is a spontaneous flow of the solvent from lower to higher concentration due to osmosis. The pressure that must be applied on the solution of higher concentration to prevent the flow of the solvent from the solution of lower concentration is called osmotic pressure of the solution. It is denoted by π

Osmotic pressure (π), can be expressed mathematically using the van't Hoff equation:
π= i C S T
where, i is the van't Hoff factor, C is the molar concentration of the solute, S is the solution constant (0.082 ltr.atm.K⁻mol⁻) and T is the absolute temperature in kelvins