Why does hot water freeze faster than cold

A.� This scientific conundrum has been debated since Aristotle’s time and still continues to cause dissent among experts.

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Hot water, which has to loose its heat before it can freeze, shouldn't freeze before cold water, which doesn't have to waste any time loosing heat but can get straight on with freezing. However despite this logical argument it does appear that in certain circumstances hot water will freeze before cold, and many scientists seem to go along with this view.

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Q.� What circumstances does hot water freeze quicker than cold

A.� When two bowls, one of cold and one of hot water, are placed on the shelf of a freezer several factors can lead to the one containing hot water to freeze first.

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Initially the hot water bowl will partially melt ice on the shelf below it and so has a better thermal contact with the freezing element. In addition, because cold water is denser than hot, as the water at the bottom of the hot water bowl cools it remains at the bottom.

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Because the water near the top of the bowl is still warm it has a more efficient evaporative cooling surface and so the top of the hot water bowl looses heat quicker than the bowl of cold water.

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Also, because hot water evaporates faster, there would soon be less of the hot water to freeze, shortening the freezing time.

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Of course in these circumstances the freezer is having to work harder and expend more energy than it does freezing cold water.

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Q.� What's happening to the bowl of cold water during this time

A.� It's undergoing normal convection cooling. As the water cools convection currents carry it through the entire body of water until all of it is frozen.

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Q.� What about the other way around, does cold water boil faster than hot water

A.� No although it may appear to get hotter quickly. The speed a liquid heats is dependent on the temperature difference between it and its immediate surroundings, including the flame that's heating it. So, because the cold water absorbs heat much quicker than the comparative hot water, initially it may appear to reach a hotter temperature very quickly.

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However, once the cold water reaches a certain hotter temperature the rate at which it absorbs heat slows down and so it cannot boil any quicker than the water that started out as hot and ultimately the hot water boils faster than the cold water.

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by Lisa Cardy