Salt makes ice colder because it lowers its freezing/melting point. As a matter of fact, when a solute is added to a solvent, it alters the colligative properties.
Colligative properties are such properties of solutions that depend on the concentration of solutes. Such properties are:
- Vapor pressure
- Freezing/melting point
- Boiling point
- Osmotic pressure
From these notable properties, vapor pressure is the most important one as the other three colligative properties are directly dependent on vapor pressure. It gets lowered when the solvent is subjected to any solute and this lowering continues as the solute is added.
Along with vapor pressure, the freezing/melting point of solutions also lowers and keeps on lowering as the concentration of solute is increased. The opposite goes for boiling point, it increases as the concentration of solute is increased. Osmotic and vapor pressures are almost the same with a difference in way of establishment of equilibrium between liquid and vapors.
Graph for depression of freezing point:
So, salts are added to ice because this causes the lowering of vapor pressure and thus depression of freezing point for ice.
This phenomenon is governed by Raoult’s law. It states that when an impurity is added to a pure solvent, it starts interacting with the solvent molecules i.e. the vapor pressure is not the same as before but reduced.
P(solution) = Xsolvent x P°solvent
X = molefraction
P = Vapour pressure