Colligative properties are a collective group of properties unique to solutions. They are solely dependent on the number of solute molecules present in the solvent; effectively the solute:solvent ratio. Introduction of solute to a pure solvent will effectively lower its freezing point.
Freezing point depression describes the phenomena in which adding solute to a solvent or the mixing of two solids results in the lowering of the freezing point of the solvent or original solids. At the freezing point, molecules pass between the two phases at equal rates. When a solute is introduced to a liquid solvent and its solid in equilibrium, the solute effectively reduces the mole fraction of the liquid solvent. This change decreases the tendency of the molecules to escape from the solvent phase, not only into the air as gas, but into the solid phase. The introduction of a solute does not affect the escape of molecules from the solid into liquid or gas. Therefore, the system is no longer in equilibrium and the solid will begin to melt. To prevent the solid from melting, the temperature should be lowered. This not only prevents the solid from melting, but also lowers the escaping tendency of the molecules from the solvent. Therefore, at a new lower temperature, there exists a temperature where the two materials can coexist in both phases in equilibrium.
The freezing point depression can also be explained in terms of vapor pressure. Addition of solute to a solvent will essentially dilute the solvent molecules, and according to Raoult's law this leads to a decrease in vapor pressure. Considering the fact that the vapor pressure of the solid and liquid forms must be the same at freezing point (or else the system would not be at equilibrium), the lowering of the vapor pressure leads to the lowering of the temperature at which the vapor pressures of the liquid and frozen forms of the solution will be equal.
The freezing point depression can be calculated by the formula: