Sunday, November 3, 2013
Have scientists worked out why hot water freezes faster than cold water?
It is a phenomenon that has baffled the world’s brightest minds since the time of Aristotle.
Now a team of physicists believe they may have solved the centuries old mystery of why hot water freezes faster than cold water.
Known as the Mpemba effect, water behaves unlike most other liquids by freezing into a solid more rapidly from a heated state than from room temperature.
Scientists have suggested dozens of theories for why this may occur, but none have been able to satisfactorily explain this strange physical property.
A team of physicists at the Nanyang Technological University in Singapore have now published what they believe may be the solution.
They claim that the explanation lies in the unusual interaction between the molecules of water.
Each water molecule is bound to its neighbour through a highly charged electromagnetic bond known as a “hydrogen bond”.
It is this that produces surface tension in water and also gives it a higher than expected boiling point compared to other liquids.
However, Dr Sun Changqing and Dr Xi Zhang from Nanyang Technological University, argue this also determines the way water molecules store and release energy.
They argue that the rate at which energy is released varies with the initial state of the water and so calculate that hot water is able to release energy faster when it is placed into a freezer.
Dr Changqing said: “The processes and the rate of energy release from water vary intrinsically with the initial energy state of the sources.”
The Mpemba effect is named after a Tanzanian student called Erasto Mpemba, who observed that hot ice cream mix froze before the cold mix.
Together with a physics professor at University College at Dar es Salaam, he published a paper in 1969 that showed equal volumes of boiling water and cold in similar containers would freeze at different times, with the hot water freezing first.
Similar observations have been described in the past, however, by Aristotle, Francis Bacon and Rene Descartes.
The effect can also have some real world implications, such as whether to use boiling water to defrost the windscreen of your car on a winters day and whether hot water pipes are more prone to freezing than cold ones.
Some people deny that the effect exists at all and is in fact an artefact of experimental procedure, but others claim to have shown it using carefully controlled experiments.
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Author: By Richard Gray, Science Correspondent
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