Which freezes first, hot or cold water?

(Lansing State Journal, March 25, 1992)

Question submitted by: Robert Iorio of Howell

Contrary to popular opinion, it is possible to have hot water freeze first.

If we put two containers with equal amounts of water into the freezer, one hot and one cold, they will both begin loosing heat to the freezer.

Normally we would expect the cold water to freeze first, because it has less heat to loose before it reaches the freezing temperature.

However, the water can evaporate, and the hot water will evaporate more rapidly than the cold.  This is because the molecules in the hot water have more energy than the molecules in the cold water and are moving faster.

Because of the evaporation, in the end, there is less water in the container that originally contained hot water.  Therefore, there’s less heat to lose in that container and the water freezes first.

The chances of seeing this effect are best if the hot water is very hot, perhaps near boiling.  Then there will be a lot of evaporation and the difference in volume between the two containers will be large.

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