We throw salt on the ground to prevent ice from forming. Ice does not form because salt lowers the freezing temperature of water. But how does it do this?
When the temperature lowers, molecules in pure water slow down. Eventually. the molecules move so slowly that they can no longer escape the intermolecular attractions between water molecules. As a result of these forces, a lattice of water molecules form and water becomes ice.
|At the freezing temperature of pure water, molecules enter and leave the solid at the same rate. Salt disrupts this equilibrium. Adapted from Silberberg’s Chemistry text.|
During this phase change, water molecules enter and leave the solid at the same rate. Salt disrupts this equilibrium by simply being present. With the addition of salt, less water molecules are present at the interface between liquid and solid. In other words, salt particles block the water molecules from re-entering the solid phase, so more water molecules are leaving and less are entering the solid. When the temperature lowers even further, the water molecules leaving the solid phase will slow down even further and the rate will eventually match the rate at which water molecules can find the solid in the presence of salt. When the rate at which water leaves the solid balances the rate at which water molecules enter, a new (lower) freezing point is established.