Hot water freezes faster than cold water: one of the oldest mysteries of physics has been deciphered

This odd phenomenon has been observed since ancient times; in 1960, it was named the Mpemba effect, after a Tanzanian student – Erasto Mpemba – who, following a cooking class, observed that ice cream mixture solidifies faster if it is placed in the freezer while warm.

Until now, no one could convincingly explain why this happens. Recently, however, a team of researchers from Singapore has come up with a plausible explanation for this strange phenomenon.

It’s all about the unusual properties of the connections that hold together the atoms forming water molecules.

A hot molecule of water is consisting of an oxygen atom linked together with two hydrogen atoms through covalent bonds.

However, when a hydrogen atom in a molecule comes close to an oxygen atom of another molecule, they bind together through a so-called hydrogen bonding.

Although hydrogen bonds are generally weaker than covalent ones, they are still stronger than Van Der Waals forces, which represents the sum of all the forces of attraction between the molecules, except the covalent bonds.

Chemists have suspected for a long time that the hydrogen bonds that tightly connect with each other the water molecules, are those that confer water some of its bizarre properties and make the boiling point of it to be higher than any other liquids composed of similar molecules.

Singapore’s researchers believe that these hydrogen bonds explain the Mpemba effect because they bring the water molecules in close contact with each other, which determines the appearance of a repulsive force between molecules. As a result, covalent bonds stretch and accumulate energy.

As the liquid is heated, the water molecules are moving away from each other, “firing” the hydrogen bonds. When molecules decrease again and release energy, this process has the effect of cooling, which makes hot water cool faster than cold water and explains Mpemba effect.

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