Q: Salt is supposed to help melt ice on the roads. When you make ice cream, you add a liberal amount of salt or rock salt to help freeze the cream. Why does the salt used in each case assist the process?
-- Scott Thomas, Tucker
A: In each case, one of the colligative properties of water -- freezing point depression -- is used, Wayne Suggs, a chemistry professor at the University of Georgia, told Q&A on the News in an e-mail. When you add a salt like sodium chloride to pure water, the temperature at which this new solution freezes will be lower than 32 degrees Fahrenheit, and the new freezing point will depend upon how much sodium chloride is added. When you add salt to ice on the road, the ice begins to melt, forming a solution below the normal freezing point of 32 degrees Fahrenheit. In the case of using salt in an ice cream churn, Suggs noted that a glass of ice water is at about 32 degrees Fahrenheit after it sits a minute. Both the ice cubes and the liquid water are at the same temperature. If salt is added to this ice water, the freezing point of the liquid water is lowered. "The liquid water loses enough heat to drop its temperature to the new freezing point, and this heat melts some of the ice," he wrote. "The ice cream mixture in the churn is initially at a higher temperature than either the ice or salt water, so heat moves into the ice and salt-water mixture, and that heat will melt more ice." In effect, the salt lowers the temperature of the water to a point where the ice cream will freeze.
Lori Johnston wrote this column. Do you have a question about the news? We’ll try to get the answer. Call 404-222-2002 or e-mail q&a@ajc.com (include name, phone and city).
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