Physics

Why does hot water freeze more quickly than cold water?

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Hot water does not, in general, freeze more quickly than cold water. However under certain circumstances this can happen. This has been the subject of a very large amount of generally poorly conducted research.

There are (at least) two mechanisms by which this can happen. Here is one of them.

Liquid water and air are both very good thermal insulators. If there is no convection in the liquid, then it takes a very long time for heat to flow from the ‘inside’ of the water to the outside.

Additionally if there is no convection above the surface of the water, then heat transfer to the air can be very slow.

Above a hot water dish open to the air, strong convection currents can be established both in the air and the water causing heat to be lost more rapidly than when the air stays still above the still liquid.