Freezing Point Depression and Boiling Point Elevation

When a non-volatile solute is added to a pure solvent, it results in the lowering of the freezing point of the solvent. This phenomenon is called freezing point depression. The extent to which the freezing point is lowered depends on the molality of the solute -the number of moles of solute per kilogram of solvent and the cryoscopic constant of the solvent.

From the plot of chemical potential, μ, against temperature, it is evident that the μ of both solid and liquid solvents decrease with temperature. Here, the intersection point, where the liquid's μ value surpasses that of the solid, indicates the freezing point of the pure solvent. When a solute is added, it reduces the μ of the solvent but does not affect the μ of the solid. This shifts the intersection point to the left, resulting in a lower freezing point for the solution than the pure solvent. For example, spreading salt on icy roads lowers the chemical potential of water in the liquid phase, depressing the freezing point and slowing ice formation.

Similarly, when a non-volatile solute is added to a solvent, it increases the solvent's boiling point. For instance, adding salt to boiling water increases the chemical potential of water in the liquid phase; the solution requires a higher temperature to reach the chemical potential needed for vaporization. This change can be calculated by multiplying the solute's molality by the solvent's ebullioscopic constant or boiling point constant, which is specific for each solvent type that can be determined either by experiment or by theoretically deriving it from the heat of vaporization for that solvent.

The addition of the solute reduces the μ of the solvent but does not affect the μ of the vapor form of the solvent. This shifts the intersection point in the μ against the temperature plot to the right, thereby raising the boiling point of the solution compared to the pure solvent.

In summary, a solute alters the chemical potential of a liquid because it disrupts the liquid’s intermolecular interactions, but gases and solids show negligible changes in chemical potential under comparable conditions.