Colligative Properties
Colligative properties are physical properties of solutions that depend only on the number of solute particles dissolved, not on the chemical identity of those particles. These properties include boiling point elevation, freezing point depression, vapour pressure lowering, and osmotic pressure. They are widely used in industries such as food preservation, antifreeze formulation, and clinical medicine to control solution behaviour.
The Four Colligative Properties of Solutions
| Property | Symbol/Formula | Depends On | Common Application |
|---|---|---|---|
| Vapour Pressure Lowering | ΔP = xₛ·P° | Mole fraction of solute | Raoult's Law calculations |
| Boiling Point Elevation | ΔTb = Kb·m·i | Molality and van't Hoff factor | Antifreeze, cooking |
| Freezing Point Depression | ΔTf = Kf·m·i | Molality and van't Hoff factor | Road de-icing, antifreeze |
| Osmotic Pressure | π = iMRT | Molarity and temperature | Dialysis, IV fluids |
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Related Terms
Boiling Point Elevation
Boiling point elevation is the phenomenon by which the boiling point of a solution is higher than that of the pure solvent, due to the presence of dissolved solute particles lowering the vapour pressure of the solvent. The increase in boiling point is directly proportional to the molal concentration of solute particles. This principle is exploited in automotive antifreeze formulations and in certain cooking techniques to raise the boiling temperature of water.
Freezing Point Depression
Freezing point depression is the decrease in the freezing (solidification) point of a solvent caused by dissolving a solute, because the solute particles disrupt the formation of the ordered solid lattice. The magnitude of depression depends on the number of solute particles per unit mass of solvent, not their chemical nature. Practical applications include road de-icing with salt, antifreeze in vehicle radiators, and the preservation of biological samples in cryoprotective solutions.
Osmotic Pressure
Osmotic pressure is the minimum pressure that must be applied to a solution to prevent the inward flow of its pure solvent across a semipermeable membrane separating the solution from the solvent. It arises because solvent molecules move spontaneously from a region of lower solute concentration (higher solvent chemical potential) to higher concentration by osmosis. Osmotic pressure is critical in biological systems — it maintains cell turgor, governs kidney function, and is the basis of reverse osmosis water purification.
From Latin "colligatus" meaning "bound together" or "collected", referring to properties collectively determined by particle count rather than identity. The term was formalised in physical chemistry in the late 19th century.