What's the Story with Ice?
14 January 1998
We fall on it, our shrubs and trees droop from it, and our vehicles slide on it: ice!
Diane Bunce, an associate professor of chemistry at The Catholic University of America, Washington, D.C., has the following to say about ice. She can be reached for interviews at 202-319-5390 and 410-451-7343.
Why does salt melt ice?
Solutions of water and dissolved ionic substances, like salt, melt at a lower temperature than water alone. This is called freezing point depression.
When you put salt - usually calcium chloride (CaCl2 ) not sodium chloride (NaCL) - on ice, the salt has such a strong attraction for the water molecules of the ice that it actually dissolves in the top layer of ice, resulting in a solution of water and calcium chloride that doesn't freeze at 32¼ Fahrenheit, but rather at a lower temperature.
Why does ice feel slippery? Ice often feels slippery to the touch because the heat from our hands actually melts the top layer of ice. What we sense as "slippery" is actually a thin coating of liquid water on the ice. This phenomenon is also called into play during ice skating. When a skater stands up on his or her skates, the entire weight of the person's body is concentrated on the edge of the thin skate blade. The blade creates such pressure on the ice surface that it causes the ice to melt and form liquid in the blade's track. The skater is actually skating on liquid water.
Water is Weird! Water has some very unusual properties, such as expanding when it freezes and compressing as it melts. This behavior is opposite to that of most substances, which contract during freezing. The reason water behaves so differently is because of the elements that it's made of - oxygen and hydrogen. A force called hydrogen bonding exists between water molecules that doesn't often occur in other substances. Hydrogen bonding causes the crystal of water to take up more room than the liquid form.
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Revised: 14 January 1998
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