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Scientists Solved The Puzzle of Why Hot Water Freezes Faster Than Cold Water

Back in this 2008 post on Neatorama, 5 Really Weird Things About Water, we told you that hot water freezes faster than cold water.

That strange effect was first observed in 1963 by a Tanzanian high school student named Erasto B. Mpemba, who was freezing hot ice cream mix in a cooking class when he noticed that hot mix froze faster than a cold one. No scientist could explain the strange behavior, dubbed "the Mpemba effect" ... until now.

Xi Zhang of Nanyang Technological University in Singapore and colleagues discovered the scientific basis of the Mpemba effect: it's because of all the hydrogen bonds that's goin' on in water.

Medium explains it in plain English for non-physicists:

Xi and co say hydrogen bonds also explain the Mpemba effect. Their key idea is that hydrogen bonds bring water molecules into close contact and when this happens the natural repulsion between the molecules causes the covalent O-H bonds to stretch and store energy.

But as the liquid warms up, it forces the hydrogen bonds to stretch and the water molecules sit further apart. This allows the covalent molecules to shrink again and give up their energy. The important point is that this process in which the covalent bonds give up energy is equivalent to cooling.

In fact, the effect is additional to the conventional process of cooling. So warm water ought to cool faster than cold water, they say. And that’s exactly what is observed in the Mpemba effect.


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The statement that "...hot water freezes faster than cold water," is misleading. It implies that it is universally true. It would be more accurate to say "Hot water can freeze faster than cold water under very specific conditions."

Those specific circumstances are outlined by the winner of the Mpemba Competition on the Royal Society of Chemistry website here: http://www.rsc.org/mpemba-competition/mpemba-winner.asp
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