Why does hot water freeze faster than cold water?

Scientists have been able to calculate that hot water actually freezes faster in a freezer than colder water under certain circumstances, as hot water transfers its energy exponentially faster.

It may sound as a paradox, but it can be confirmed even with a simple experiment at home: hot water freezes faster than water at room temperature. This fact has been observed since ancient times and by famous scientists such as Aristotle or Descartes, but it is called the Mpempa phenomenon in honor of Erasto Mpempa, a student from Tanzania who described it in 1963.

Over time, scientists have proposed dozens of explanations for the phenomenon, which seems to contradict the basic laws of thermodynamics, but no theory has been adopted as the definitive answer. In fact, in 2012, the Royal Society of Chemistry of Great Britain announced that it would give a prize of 1,000 pounds to anyone who managed to satisfactorily explain the phenomenon, in a competition where 22,000 people participated. Although the award was given to Nikola Brekovic, the issue was not considered to be fully settled.

Two physicists from Singapore's Nanyang Polytechnic, however, believe they have come up with a comprehensive explanation of the phenomenon, which takes into account the particular interactions between water molecules.

Each water molecule binds to its neighboring ones with an electromagnetic bond called a hydrogen bond. In fact, it is responsible not only for the high surface tension of water, but also for the high boiling point of water compared to other liquids.

Dr. Sun Changqing and Dr. Xi Zhang went a step further, considering that hydrogen bonds also regulate the way water stores and releases energy. According to them, the rate of energy change in water molecules depends on its initial state.

In particular, when water is heated, hydrogen bonds interact with covalent bonds that are also involved in water molecules, causing the latter to shrink, storing energy when heated. This contraction during heating, leads to the release of energy at an exponentially faster rate during cooling, compared to the colder water in which the covalent bonds are not compressed. With this in mind they were able to calculate how hot water actually freezes faster in a freezer than colder water under certain circumstances, as hot water transfers its energy exponentially faster.

The research of the two scientists was published in the journal Scientific Reports.

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Source: naftemporiki.gr

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