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The four classical colligative properties are vapor-pressure lowering, boiling-point elevation, freezing-point depression, and osmotic pressure. For ideal dilute solutions, Raoult’s law relates relative vapor-pressure lowering to the solute’s mole fraction. Boiling-point elevation and freezing-point depression vary with molality and the van ’t Hoff factor; osmotic pressure follows an ideal-gas-type relationship. The van ’t Hoff factor reflects particle changes such as electrolyte dissociation or solute association. Real solutions may depart from ideal behavior, particularly when concentration or solute–solvent interactions are significant. These principles help explain phase-diagram shifts and support applications including osmometry, antifreeze formulations, reverse osmosis, and tonometry.