Boiling Point Elevation

The effect on the boiling point is just the opposite; that is, the boiling point of a liquid is increased if something is dissolved in it.

Again, the antifreeze in a car radiator is an example of this. In the advertisements it's called summer protection against boil-overs. Essentially what they have done is raised the boiling point of the liquid, of the water, by making it a solution.

Antifreeze label showing higher boiling points.

One way of summarizing both freezing point depression and boiling point elevation is to say that the addition of solute extends the temperature range over which the liquid can exist. It will boil at a higher temperature and it will freeze at a lower temperature. Note that as the concentration of the solute increases, so does the effect on both the freezing point and the boiling point.

Thermometers showing expanded temperature range of solutions.

A very practical use of this phenomenon was mentioned earlier - antifreeze. Almost anything can be used to lower the freezing point of water, or to raise the boiling point, as long as it dissolves in water. But there are some very important, practical limitations. For example, if you have a very volatile material such as ethanol, it would not work well in a car's radiator, because as the engine got hot the ethanol could vaporize and leave; it would no longer be dissolved in the solution, therefore it would not be able to lower the freezing point of the solution. On the other hand an ionic material would work but it would probably enhance the corrosion of the interior of the car's engine. You don't want that to happen either. So you have to be very careful about the other physical and chemical properties of the material that you choose when you want to lower the freezing point or raise the boiling point for some particular purpose.

In general, the extent of the boiling or freezing point change is proportional to the number of moles of solute added to a certain amount of water, regardless of what specific chemical is used.

Electrolyte Effect

However, you have to take into account the degree of dissociation of the solute. If a strong electrolyte dissociates into two ions, it will be twice as effective at changing the boiling or freezing point. If a strong electrolyte dissociates into three ions, then it is three times as effective as the same number of moles of a nonelectrolyte. If a weak electrolyte dissociates 5% then it would be 5% more effective than an equal number of moles of a nonelectrolyte.

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E-mail instructor: Eden Francis

Clackamas Community College
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