Many have heard the "old wives tale" stating that if you put hot water into a freezer, it freezes in less time than it would have if it had been cold to start. Never did I believe such a claim, as it runs contrary to one of the basic laws of nature.

While surfing one day on sites illustrating "bad science" I actually found a plausible real life reason why this in fact can be true (read:not always true, but possible). Running this experiment under controlled conditions [eg. measure the same volumes of hot and cold water, make sure containers are equal in any relevant aspect (shape, material, conductance properties, covered or not etc.), and that the freezer is properly set to achieve a uniform temperature throughout], can you come up with a reasonable set of conditions for which the water in the hot container would freeze before the water in the cold container?

If both the cold and hot water are identical in composition, then the cold water will freeze faster due simply to the laws of heat transfer. Most people believe the opposite based on experience.
When water is boiled until all the trapped air bubbles are released, it will freeze faster than cold water. Not because it's hot, but because of the different composition. Heating water promotes evaporation.
So, some of the mass is lost and there is actually less water to be cooled.
Second, during evaporation, the hottest molecules are released into the air first, actually lowering the temperature of the remaining water (that's why blowing on hot soup makes it cool off).

The folklore about hot water freezing faster probably started in the nineteenth century when water was carried in wooden buckets, which help retain the heat and increase evaporation. A modern experiment using wooden buckets showed hot water freezing 10% faster then a bucket of water at room temperature. (Bill McLain, "Do Fish Drink Water?")

Posted by DJ on 2003-05-11 13:45:38