26 February 2015

Why hot water freezes faster than cold water - called the Mpemba Effect

We've Finally Figured Out Why Hot Water Freezes Faster Than Cold
Jamie Condliffe

For centuries, scientists have puzzled over a counter-intuitive observation: hot water, for some reason, seems to freeze faster than cold. Fortunately, now a team of physicists has worked out why it happens.
Known as the Mpemba effect—after a Tanzanian student who noticed that hot ice cream mix freezes faster than a cold—it was in fact first observed by Aristotle, then later Francis Bacon and René Descartes. But while it's been observed, recorded, and discussed by eminent thinkers for years, nobody has ever worked out why hot water freezes more quickly than cold. Enter Xi Zhang and his colleagues from the Nanyang Technological University in Singapore. They've found evidence which suggests that it's the chemical bonds that hold water together which provide the strange effect. 
First, some chemistry. Each molecule of water is made up of two hydrogen atoms bonded covalently to a single atom of oxygen. Those bonds, which involve atoms sharing electrons, are well understood. But the separate water molecules are bound together, too, by weaker forces generated by hydrogen bonds. They occur when a hydrogen atom from one molecule of water sits close to an oxygen atom from another—and they give rise to many of water's interesting properties, like its strangely high boiling point.
Now, Xi Zhang is suggesting that those same bonds cause the Mpemba effect. The idea is pretty simple: bring water molecules into close contact, and a natural repulsion between the molecules causes the covalent bonds to stretch and store energy. As the liquid warms up, the hydrogen bonds stretch as the water gets less dense and the molecules move further apart.
That extra stretch in hydrogen bonds allows the covalent bonds to relax and shrink a little, giving up their energy. The process of covalent bonds giving up energy is equivalent to cooling, so warm water should in theory cool faster than cold. Which is the Mpemba effect!
Xi Zhang's theoretical calculations suggest that the magnitude of the covalent bond relaxation exactly accounts for the experimental differences in the time it takes for hot and cold water to freeze—which is, frankly, amazing. It's worth noting that the work isn't peer-reviewed yet, but it does seem to neatly explain the theory. And considering nobody else has a decent explanation, it seems likely to be right. [arXiv via Medium]

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