freezing-point depression

Scatter salt on an icy path and the ice turns to slush even though the air stays below freezing. The salt has lowered the temperature at which water can freeze. Freezing-point depression is this effect: dissolving a solute makes a liquid freeze at a colder temperature than the pure solvent would.

Freezing happens when molecules lock into an orderly solid. Dissolved solute particles get in the way of this orderly packing — they crowd the liquid and dilute the solvent molecules that are trying to crystallize — so the liquid must be cooled further before it can freeze. Like the boiling-point rise, it is a colligative property: the drop in freezing point depends only on how many solute particles are dissolved, found from the molality times a constant set by the solvent.

This is why salt clears winter roads, why antifreeze keeps a car's coolant liquid in deep cold, and why a pinch of salt helps an ice-cream maker get colder than plain ice. It also gives a sensitive lab method, cryoscopy, for measuring molar masses: weigh a sample, dissolve it, and read how far the freezing point falls.