Why does hot water freeze more quickly than cold water?

You might have tried to freeze water in your fridge. But have you ever noticed that hot water freezes faster than cold water? If not, try it yourself.

This phenomenon where hot water freezes faster than cold water is called Mpemba effect. The Mpemba effect has been a well known physicist since the time of Aristotle who noticed it more than 2,000 years ago. There were many ideas since then to have been put forward to explain the same. One idea is that warm containers make better thermal contact with a refrigerator and so conducts heat more efficiently, which leads to faster freezing. Another idea is that warm water evaporates rapidly and since this is an endothermic process, it cools the water making it freeze more quickly. Scientists have observed the Mpemba effect in experiments which utilizes closed containers where evaporation rate is zero. Back in 2012, the Royal Society of Chemistry ran a competition asking scientists to explain the phenomenon, and despite receiving some 22,000 papers from all over the world, none of the explanations were convincing enough on their own to draw widespread consensus. Another theoretical speculation is that water develops convection currents and temperature gradients as it cools.

If we consider a glass of hot water which is undergoing rapid cooling, it will have a higher temperature difference throughout, and it loses heat energy faster from the surface when compared to a uniformly cool glass of water. A uniformly cool glass of water has a lower temperature difference and also the amount of convection is less which can accelerate the process. But this idea has not been verified either.

Francis Bacon and René Descartes also made similar observations. After that, the possibility of hot water freezing faster than cold water finally gained more acceptances in the 1960s. Thanks to a Tanzanian schoolboy, Erasto Mpemba (after whom the phenomenon was hence named) who noticed the effect when making ice cream.

Erasto Mpemba and his schoolmates used to make ice cream by boiling milk and mixing it with sugar, and letting it cool before placing it in the freezer. One day Mpemba became frustrated and lost his patience, and then instead of allowing his icecream mix to cool down, he placed the boiling milk into the freezer immediately. He found that his ice cream became set faster than his friends. Later in 1969, Mpemba published a paper which describes this phenomenon with the help of a physics professor.

Today Xi Zhang at the Nanyang Technological University in Singapore and a few of his colleagues are trying to provide a plausible explanation for the same. These researchers say that the Mpemba paradox is the result of the unique properties of the bonds which can hold water molecules together. They proposed that the hydrogen bonds bring water molecules together and when they are nearby, the repulsion between water molecules causes the O-H bonds to stretch and energy gets stored.

But when the liquid gets warmed up, the hydrogen bonds tend to stretch and the water molecules stay far apart. This process helps the covalent molecules to compress again and lose their energy. It starts cooling when they lose energy. Hence, warming of water would initiate and enhance cooling as the bonds give up energy.
In their paper they said that the analysis led them to suggest a molecular explanation for the Mpemba effect. They proposed that, in warm water, the weaker H-bonds which have predominantly electrostatic contributions, are broken, and smaller clusters of water with strong Hydrogen bonding arrangements exist. This accelerates the nucleation process which leads to the formation of hexagonal lattice of solid ice. Therefore they say that warm water freezes faster than cold water. In fact, the effect is additional to the conventional process of cooling. So according to them, warm water cools down faster than cold water. And that’s exactly what we observe in the Mpemba effect.

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