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What are colligative properties?

Class 12 ChemistryNCERT chapter: Solutionsइसे हिंदी में पढ़ें

Exam-ready answer

Colligative properties are properties of dilute solutions that depend on the number of solute particles relative to the total number of particles in the solution, irrespective of the nature of the solute. The four colligative properties are relative lowering of vapour pressure, elevation of boiling point (ΔT_b = K_b m), depression of freezing point (ΔT_f = K_f m) and osmotic pressure (π = CRT).

Explanation

When a non-volatile solute dissolves in a solvent, its particles occupy part of the surface, so fewer solvent molecules escape and the vapour pressure falls. By Raoult's law, (p₁° − p₁)/p₁° = x₂, the mole fraction of the solute. This lowering of vapour pressure is the root of the other effects. It depends only on how many solute particles are present, whether they are glucose molecules, urea molecules or ions.

A liquid boils when its vapour pressure equals the atmospheric pressure. Since the solution's vapour pressure is lower, it must be heated to a higher temperature to boil: ΔT_b = K_b m, where m is molality and K_b is the ebullioscopic constant in K kg mol⁻¹. Similarly, the solution freezes at a lower temperature: ΔT_f = K_f m, with K_f the cryoscopic constant. Spreading salt on icy roads works this way: the salt water freezes below 0 °C, so the ice melts.

Osmotic pressure is the extra pressure that must be applied to a solution to just stop solvent flowing into it through a semipermeable membrane. For dilute solutions π = CRT, where C is molar concentration, R the gas constant and T the temperature. Because the effects depend on particle number, colligative properties are used to find molar masses; osmotic pressure suits proteins and polymers. Electrolytes give abnormal values, corrected using the van't Hoff factor i.

Points to remember

Colligative properties depend only on the number of solute particles, not on their nature.
Relative lowering of vapour pressure: (p₁° − p₁)/p₁° = x₂ (mole fraction of solute).
Elevation of boiling point: ΔT_b = K_b m; depression of freezing point: ΔT_f = K_f m (K_b, K_f in K kg mol⁻¹).
Osmotic pressure: π = CRT, where C is molarity; useful for molar mass of macromolecules.
Van't Hoff factor i corrects for dissociation or association, e.g., ΔT_f = i K_f m.

Exam tip

Name all four properties and write each formula with its symbols and units: ΔT_b = K_b m and ΔT_f = K_f m with K kg mol⁻¹, π = CRT. In numericals, convert grams to moles and solvent mass to kg before finding molality, and apply i for electrolytes.

Questions students also ask

Why does salt melt ice on roads in cold places?

Salt dissolves in the thin layer of water on ice, forming a solution. Dissolved ions lower the freezing point of water below 0 °C, which is depression of freezing point. At the existing temperature the ice can no longer stay frozen in contact with the solution, so it melts.

Why does 0.1 m NaCl show about twice the freezing point depression of 0.1 m glucose?

Glucose stays as whole molecules, but NaCl dissociates into Na⁺ and Cl⁻ ions in water. So 0.1 m NaCl produces nearly 0.2 m of particles. Since colligative properties depend on the number of particles, the depression is almost doubled; the van't Hoff factor i is close to 2.

Why is osmotic pressure preferred for finding molar masses of proteins?

Proteins and polymers have very high molar masses, so their solutions contain few particles and give tiny changes in boiling or freezing point. Osmotic pressure is measurable even then, is measured at room temperature, and uses molarity, so it suits biomolecules that are unstable at high temperatures.

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