Scientists claim to have solved why hot water appears to freeze 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.
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.
There are a number of theories for might cause this, including that evaporation of hot water means there is less water to freeze.
Another theory suggests that dissolved gasses in the water are released in hot water and so make it more viscous.
Last year the Royal Society of Chemistry offered a £1,000 prize to anyone who could explain how the Mpemba effect worked.
Nikola Bregovic, a chemistry research assistant at the University of Zagreb, was announced as the winner for the prize earlier this year.
He conducted experiments using beakers of water in his laboratory and his resulting paper suggested that the effect of convection was probably responsible.
He said that convection currents set up in the warm water cause it to cool more rapidly.
However, Dr Changqing and Dr Zhang have attempted to explain the effect further by examining the process at a molecular level.
Last week they published a paper in the journal Scientific Reportsshowing how water molecules arrange themselves when forming ice
They also published a paper on arXiv Chemical Physics that explained the Mpemba effect.
They say the interaction between the hydrogen bonds and the stronger bonds that hold the hydrogen and oxygen atoms in each molecule together, known as covalent bonds, is what causes the effect.
Normally when a liquid is heated, the covalent bonds between atoms stretch and store energy.
The scientists argue that in water, the hydrogen bonds produce an unusual effect that causes the covalent bonds to shorten and store energy when heated.
This they say leads to the bonds to release their energy in an exponential way compared to the initial amount stored when they are cooled in a freezer.
So hot water will lose more energy faster than cool water.
Dr Changqing said: “Heating stores energy by shortening and stiffening the H-O covalent bond.
“Cooling in a refrigerator, the H-O bond releases its energy at a rate that depends exponentially on the initially stored energy, and therefore, Mpemba effect happens.”
The Royal Society of Chemistry received more than 22,000 responses to its call for a solution to the Mpemba effect and it is still receiving theories despite the competition closing a year ago.
Mr Bregovic, who was judged to have developed the best solution by a panel of experts a conference at Imperial College London last year, said: “This small simple molecule amazes and intrigues us with its magic.”
Aeneas Wiener, from Imperial College who helped to judge the competition, added: “The new paper demonstrates that even though a phenomenon seems simple, delving deeper reveals even more complexity – and that is certainly worth looking at.
“We hope it’ll inspire young people to pursue scientific studies.”
Dr Denis Osborne, a lecturer at University College in Dar es Salaam who published the paper with Mr Mpemba on the effect they had observed, said: “Several different mechanisms may cause or contribute to an Mpemba effect.
“What the authors describe as a property of H-O bonding may be one of these.”