Colligative properties

As a colligative property (from Latin colligere "collect" ) is referred to in chemistry with a material property, which depends only on the number of particles ( moles ), but not dependent on the nature of the particles or their chemical composition.

Examples of this are the phenomena of

• Lowering of vapor pressure
• Freezing point depression ( cryoscopic )
• Boiling point elevation (ebullioscopy, see Cryoscopy )
• Osmotic pressure

These so-called colligative phenomena occur in dilute solutions and depend only on the mole fraction of the dissolved component.

The statement that the colligative properties depend only on the number of particles of solute and not of the kind is wrong, because this statement is only true for solutes that have a negligible vapor pressure relative to the solvent, eg for salts, which are dissolved in water. For a solution of ethanol in water, this relationship is not true, because ethanol has in comparison to water is not negligible vapor pressure.

Cause of the freezing point depression and boiling point elevation is that the dissolved substances by reducing their entropy of mixing the chemical potential of the solvent.