Freezing Point Depression

Freezing point depression is not just another way of referring to the early February blues. Instead, it has to do with the change that occurs in the temperature at which a liquid freezes (or a solid melts) when a solute is dissolved in it.

For example, consider the effect that salt has on the melting point of ice (which is the same as the freezing point of water). In the Midwest, salting of roads is very common during the winter; it melts the ice and snow that's present on the roads. Here in the Northwest, sanding is more common than salting for roads but some people will use salt on sidewalks as does the College.

Another example of this is the use of antifreeze in car radiators. By using a solution instead of pure water in the radiator, the liquid will not freeze until you get to some temperature below 0°C (which is 32°F), rather than freezing right at 0°C.

The next time you see a container of antifreeze, look on the label and it will show you that as you increase the amount (concentration, actually) of the antifreeze in the car's cooling system the freezing point of the solution decreases. There is a limit to that, and the instructions usually say that you should not exceed a certain percentage. If you get to the point where you have water in the antifreeze rather than antifreeze in the water, the freezing point starts going back up--although it is still very low.

Antifreeze label showing lower freezing points.

In general, for dilute solutions, the amount of change in the freezing point is proportional to the concentration of the solute in the solution.

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E-mail instructor: Eden Francis

Clackamas Community College
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