freezing point depression
Why does salt melt ice? In pure water, at 0°C, ice melts just as fast as water freezes. You won't see any of the ice melt as long as the freezing rate and melting rates are exactly equal .
Adding salt (or any foreign substance) to the water upsets the delicate balance between freezing and melting. Fewer water molecules reach the surface of the ice in a given time, so water freezes more slowly. The melting rate isn't changed by the salt, so melting "wins" .
Does adding salt to ice and water cause a temperature drop? Yes. This is how old-fashioned ice cream makers lowered the temperature of the ice cream below water's ordinary freezing point. A mixture of rock salt, ice, and water packed in the bucket around the ice cream mix can bring the temperature down as low as -21°C.
Why does the temperature drop? Energy is required to snap the hydrogen bonds that hold the ice together. The melting ice draws that energy from the surrounding solution as heat.
If you'd like to extend your project, here are some suggestions:
"Why does salt melt ice?" (includes a Flash simulation of freezing point depression)
"How can freezing point depression be explained in terms of free energies?"
Author: Fred Senese firstname.lastname@example.org
Copyright © 1997-2010 by Fred Senese
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Last Revised 08/17/15.URL: http://antoine.frostburg.edu/chem/senese/101/solutions/faq/print-why-salt-cools-icewater.shtml