Freezing Point Depression & Boiling Point Elevation

A. Boiling Point Elevation

The boiling point of a solution is the temperature at which its vapor pressure equals the external atmospheric pressure. The presence of the nonvolatile solute lowers the the vapor pressure of a solution because the solutes literally block the solvent molecules from going into gaseous state. As a result, it takes more energy for the vapor pressure to equal the atmospheric thereby elevating the boiling point of the solution.

Therefore,

The freezing point depression can be explained in the following manner. Freezing involves a transition from the disordered states to the ordered state. For this to to happen, energy must be removed from the system. Because a solution has greater disorder than the solvent, more energy is needs to be removed from it to create order than in than in the case of a pure solvent. Therefore, the solution has a lower freezing point that its solvent. Note that when a solution freezes, the solid that separates is the pure solvent component.