Solution & Colligative properties Notes for Class 12, IIT JEE & NEET

Colligative properties are properties of solutions that depend only on the number of dissolved solute particles, not on their chemical identity. The four colligative properties in the Class 12 NCERT syllabus are: relative lowering of vapour pressure, elevation of boiling point, depression in freezing point, and osmotic pressure. Understanding these helps predict how dissolving a solute changes physical behaviour of a solvent.

Osmotic pressure is the most accurate colligative property for determining molar mass, especially of macromolecules like proteins and polymers. This is because even a small concentration produces a measurable osmotic pressure, making it highly sensitive. For smaller molecules, freezing point depression (using Kf) is preferred over boiling point elevation because the Kf value is larger, giving a more measurable temperature change.

For water: Kb (ebullioscopic constant) = 0.52 K·kg/mol and Kf (cryoscopic constant) = 1.86 K·kg/mol. These values are fixed constants for a given solvent and are independent of the solute used. The significantly larger Kf makes freezing point depression more practically useful for molar mass determination in laboratory settings.

The Van't Hoff factor (i) multiplies the colligative property value to account for dissociation or association of solute particles. For strong electrolytes like NaCl, i ≈ 2 (produces Na⁺ and Cl⁻), so the colligative property observed is nearly double the calculated value. For associating solutes like acetic acid in benzene, i < 1, so the observed property is smaller than expected.

An ideal solution obeys Raoult's Law at all concentrations and temperatures. It shows zero enthalpy of mixing (ΔH_mix = 0) and zero volume change on mixing (ΔV_mix = 0). Intermolecular forces between solute–solvent molecules (A–B) are equal to those in pure components (A–A and B–B). A classic example is benzene + toluene. Real solutions deviate positively or negatively from Raoult's Law.