Unit 6: Solubility and Redox Reactions

Section 2: Freezing Point and Boiling Point Change

* Introduction
* Freezing Point Depression
* Boiling Point Elevation

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.