Careful of the Icy Patch

In the winter time, as hardy Canadians, we pour salt onto the icy patches on the sidewalks to keep people from slipping and preventing litigation. But ever wondered why salt? What does the salt do? Well I shall explain this by introducing the concept of colligative properities.

A colligative property is one where by introducing an impurity of some sort (this is often called the solute) the properties of the solution are changed. What makes a colligative property different from other chemical properties is that a colligative property is only dependent on the number of added impurity molecules, not the type of molecules. Meaning it only matters how much you added, not what you add.

The important colligative property that we need here in Canada during the winter time is: Freezing Point Depression. When you add an impurity, such as salt or sand or antifreeze (ethylene glycol) to a solution of water (or that icy patch on your front walk) the result is that the freezing point of the water becomes lower. This means that even though it is -15 °C outside, the ice on your walkway stays liquid. And because freezing point depression is a colligative property: it doesn’t make a difference what kind of additive you put on the icy patch. All will cause the freezing point to become depressed. The amount that the freezing point lowers is solely dependent on the amount added, not what is added.

Freezing point depression is used in undergraduate chemistry labs the world over as a means to determine the purity of the products that they were supposed to isolate. If they have a pure compound then the freezing point should be nice and sharp, and agree with literature standards. If they have an impure compound then their freezing point will be much lower than the standards.

Another example of a colligative property is Boiling Point Elevation. Salt added to water will actually increase the boiling point of the water. Now, practically when adding salt to water when cooking pasta, we do this for seasoning, and though in principle the boiling point of the water will increase, we won’t observe a measurable difference-unless you like your pasta REALLY salty.

Reference:

Laidler, K. J.; Meiser, J. H.; Sanctuary, B. C. Physical Chemistry 4th ed. 2003; Houghton Mifflin Company, Boston, MA.