Freezing Point Depression and Boiling Point Elevation

When a non-volatile solute is added, it lowers a solvent’s freezing point, known as the freezing point depression. This effect is calculated as the product of the solute’s molality and the solvent’s cryoscopic constant.

The chemical potential, μ versus temperature plot shows that the μ of the solvent in both its solid and liquid states decreases with temperature.

The intersection point, where the μ of the liquid equals that of the solid, represents the freezing point of the pure solvent.

When a solute is added, the μ of the liquid solvent decreases, while the μ of the solid solvent remains unchanged, shifting the intersection to a lower temperature.

Similarly, adding a non-volatile solute to a pure solvent elevates the solution's boiling point, which can be expressed as the product of the molality of the solute and the boiling point constant of the solvent.

This change is due to the solute reducing the solvent's μ value without affecting the vapor, shifting the intersection point to the right and raising the boiling point.

Overall, adding a solute always changes the chemical potential of the liquid phase, while the chemical potentials of solids and gases remain unchanged.