Koligatif

Koligatif, in chemistry often referred to as colligative properties, describes a set of solution properties that depend on the number of solute particles present, not on their chemical identity. The magnitude of these properties increases with solute concentration and is influenced by whether solutes dissociate in solution through the van’t Hoff factor (i).

The main colligative properties are:

- Boiling point elevation: ΔTb = i Kb m, where Kb is the ebullioscopic constant and m is the molality. More particles raise the temperature required to boil the solution.

- Freezing point depression: ΔTf = i Kf m, with Kf the cryoscopic constant. More particles lower the temperature at which the solution freezes.

- Osmotic pressure: π = i M R T, where M is the molarity, R is the gas constant, and T is temperature. Solute particles generate pressure across a semipermeable membrane.

- Vapor pressure lowering: P_solution = x_solvent P°_solvent (Raoult’s law for ideal solutions). Solute presence reduces the solvent’s surface molecules and lowers the vapor pressure.

Key concepts include the van’t Hoff factor i, which reflects solute dissociation or association. For non-electrolytes, i ≈ 1; for electrolytes, i ≈ number of particles produced by dissociation (though real solutions may show deviations at higher concentrations).

Applications and examples:

- Adding salt to water lowers the freezing point, useful in de-icing and preserving icy roads.

- Ethylene glycol in antifreeze raises boiling point and lowers freezing point in engines.

- Osmotic pressure underpins phenomena in biology, such as cell t membranes and dialysis.

Limitations:

Colligative relations assume ideal or dilute solutions. At higher concentrations or with strong solute–solvent interactions, deviations occur, and the simple equations no longer fully describe behavior.