Mpemba Effect

Mpemba Effect


Hot water turns into ice faster than cold water.


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There is the strange Mpemba effect, named after a Tanzanian student who discovered that a hot ice cream mix freezes faster than a cold mix in cookery classes in the early 1960s. (In fact, the effect has been noted by many scientists throughout history including Aristotle, Francis Bacon and René Descartes.)

The Mpemba effect is the observation that warm water freezes more quickly than cold water. The effect has been measured on many occasions with many explanations put forward. One idea is that warm containers make better thermal contact with a refrigerator and so conduct heat more efficiently. Hence the faster freezing. Another is that warm water evaporates rapidly and since this is an endothermic process, it cools the water making it freeze more quickly.

None of these explanations are entirely convincing, which is why the true explanation is still up for grabs.

Xi Zhang at the Nanyang Technological University in Singapore and a few pals provide one. These guys say that the Mpemba paradox is the result of the unique properties of the different bonds that hold water together.

They 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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