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 properties".
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 molecule.
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. When you are making pasta and you add a little salt to the water, this does not make the water boil sooner (contrary to what my ninth grade science teacher taught me) it actually raises the temperature that the water boils at.
Reference:
Laidler, K. J.; Meiser, J. H.; Sanctuary, B. C. Physical Chemistry 4th ed. 2003; Houghton Mifflin Company, Boston, MA.
Reference:
Laidler, K. J.; Meiser, J. H.; Sanctuary, B. C. Physical Chemistry 4th ed. 2003; Houghton Mifflin Company, Boston, MA.
Thanks for this article which is relevant to us now in Montreal. Also thanks for the reference to our text book, Laidler, Meiser and me. I should let you know that the Authors now publish the text book, not Houghtom Mifflin. It is published as an ebook (pdf) and is the way of the (green) future. If you want to see how it works, you can get chapter 1 and the solution manuals for free:
ReplyDeleteSolution manual
http://www.mchmultimedia.com/store/chemistry-freeware.html
Chapter 1
http://www.mchmultimedia.com/store/register.php?action=try&cat=64
Best wishes
Bryan Sanctuary
So are you telling me that adding salt to my pot is going to make it take longer to boil?
ReplyDeleteYep, that is exactly what I am saying: by adding salt you are making it take longer to boil; however, the amount you add versus the amount of time you spend waiting for the pot to boil you probably won't notice a difference. When you are cooking, the salt is actually added to the pot for flavour.
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