koligative

Colligative properties, often spelled from the correct term collIgative properties, are properties of solutions that depend on the number of dissolved particles rather than their identity or chemical nature. The major colligative properties are freezing-point depression, boiling-point elevation, osmotic pressure, and vapor-pressure lowering. They arise because adding solute particles lowers the chemical potential of the solvent, reducing its activity and altering phase equilibria.

In dilute solutions, these properties are related to solute concentration and can be described using molality (m), molarity (M), mole fraction, and the van’t Hoff factor i, which accounts for solute dissociation or association. For non-dissociating solutes, i is approximately 1; for electrolytes, i reflects the number of particles formed per formula unit (e.g., i ≈ 2 for NaCl in dilute solutions). Freezing-point depression is ΔT_f = i K_f m, and boiling-point elevation is ΔT_b = i K_b m, where K_f and K_b are the solvent’s freezing and boiling point constants. Osmotic pressure is given by π = i M R T (or π = i c R T). Vapor-pressure lowering follows Raoult’s law: P_solution = x_solvent P°_solvent, with ΔP ≈ x_solute P° at low solute concentrations.

Limitations include non-ideal behavior at higher concentrations or with strong solvent–solute interactions, where i deviates from simple predictions and activity coefficients become relevant. Applications include determining molar masses via colligative measurements, designing antifreeze and de-icing formulations, and understanding biological osmotic processes.