Trivia Junction

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Hot water freezes faster then Cold water

For thousands of years, chemists, physicists and high-school science-fair participants have pondered the seemingly contrary fact that heated water freezes faster than cold water. Even the ancient Greek philosopher Aristotle noted the phenomenon.

Finding a conclusive answer as to why, however, has long been elusive. But Jonathan Katz, a professor of physics at the University of Washington at St. Louis, thinks he has the answer. The solution is solutes – trace elements or minerals dissolved in a fluid.

“You have to ask yourself,” Katz told New Scientist magazine, “what does heating do to water that makes it easier to freeze? The answer is that it precipitates out solutes.”

Specifically, it removes calcium and magnesium bicarbonate, two minerals that can make drinking water “hard.” When water is heated, Katz said, these minerals condense, forming a solid scale that can accumulate on, say, the inside of a kettle.

Conversely, unheated water is typically full of solutes. As it freezes, ice crystals form and concentrate the solutes in an ever-smaller amount of unfrozen water. Solutes lower the freezing point of water, just as salt keeps roads snow-free.

Katz told New Scientist there's a second, related effect. Because the freezing point of unheated water containing solutes has been lowered, there's less temperature difference between the liquid water and its frozen surroundings. How fast something freezes is dictated by this difference in temperature. The bigger the difference, the faster the heat loss and the quicker the rate of freezing.

“Water that has not been heated has greater difficulty losing heat,” Katz said.

Bottom line: If you need to make ice cubes fast, use distilled water.

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