The presence of a solute in a solvent lowers the freezing point and raises the boiling point of the solvent. These properties also depend on the amount of solute present; i.e., they are colligative properties. The particles of the solute interfere with the attraction between the molecules of the solvent. It is therefore necessary to bring the molecules of the solvent even closer together to exclude the solute from interfering with crystal formation in the solvent. In order to bring the molecules closer, the molecules must exhibit less motion, a characteristic of a lower temperature. The addition of a solute therefore lowers the freezing point. Since the addition of a solute lowers the vapor pressure and boiling occurs when the vapor pressure equals the atmospheric pressure, the solvent with solute requires a higher temperature to bring the solution to the boiling point than the solvent alone.
The amount of freezing point lowering is given by
where m is the molality of the solution and is a constant of proportionality dependent on the solvent. Similarly, the boiling point elevation is given by
