Definition
Colligative properties are physical properties of solutions that depend only on the number of solute particles relative to solvent particles in a solution, and not on their chemical identity or mass.
Types of Colligative Properties
| Property | Description | Example |
|---|---|---|
| Relative lowering of vapor pressure | Addition of nonvolatile solute reduces solvent vapor pressure (described by Raoult's law). | Dissolving salt or sugar in water lowers its vapor pressure compared to pure water. |
| Boiling point elevation | A solution boils at a higher temperature than the pure solvent. | Adding NaCl to water raises its boiling point above 100°C. |
| Freezing point depression | A solution freezes at a lower temperature than the pure solvent. | Ethylene glycol in car antifreeze lowers the freezing point of radiator water. |
| Osmotic pressure | The minimum pressure required to stop net solvent flow across a semipermeable membrane. | Cellular fluid balance maintained across cell membranes in biological systems. |
Everyday Examples
- Road de-icing: Salt lowers the freezing point of water, melting ice on winter roads.
- Cooking pasta: Adding salt to boiling water slightly raises the boiling point temperature.
- Antifreeze in engines: Ethylene glycol depresses freezing point in winter and elevates boiling point in summer.
- Biological balance: Osmotic pressure prevents red blood cells from swelling or shrinking (crenation/hemolysis).
Key Notes
- Colligative properties strictly hold true for ideal dilute solutions.
- For electrolytes that dissociate (e.g., NaCl → Na+ + Cl−), the total number of particles increases, requiring the van 't Hoff factor (i) correction.
- Measurement of colligative properties (especially osmotic pressure) is widely used to determine the molar mass of macromolecules and polymers.
In short: Colligative properties depend on particle count, not type.
They explain why salt melts ice, antifreeze protects engines, and osmotic pressure sustains biological life.