Section 2: Freezing Point and Boiling Point Change

Introduction

In the winter, when it snows it is an accepted practice that salt will be put on roads and sidewalks to deice them, and to make them safer. But why does this work? Why does the salt seem to melt the ice? It is all explained by something called freezing point depression.

Freezing Point Depression

As we already know, the addition of a solute affects the vapor pressure of a liquid solvent. Since changes of state depend on vapor pressure, the presence of a solute also affects the freezing point and boiling point of a solvent. Thus, with the addition of a solute to a solvent, the freezing point of the solution is lower than the freezing point of the pure solvent. This is because the vapor pressure of the newly formed solution is less than the vapor pressure of the solidified form of the solution. For example, water freezes at 0 degrees centigrade. At that temperature, the vapor pressures of the ice and the water are the same. But, when the solute is added, the vapor pressure of the liquid solution is less than that of ice, and thus it does not freeze and the freezing point is depressed.

This situation can be represented by an equation:

DeltaT= Kfmsolute

with Kf being the molal freezing point depression constant, and the msolute being the molality of the solute in the solution.

Getting back to the salt on the ice, it should also be noted that salt is not useful if the temperature gets lower than the freezing point depression, because it cannot be used under extreme cold temperatures.

Boiling Point Elevation

Boiling point elevation works along the same pricnipals as freezing point depression, except that it is on the opposite end of the spectrum. When a solute is added to a solvent, the boiling point of the new solution is raised. As with freezing point depression, the magnitude of the boiling point elevation depends upon the concentration of the solute. This can also be represented by an equation that is similar to that of freezing point depression:

DeltaT=Kb msolute

with Kb being the molal boiling point elevation constant and msolute being the molality of the solute in the solution.