Why is hot water freezing faster than cold water?

Researchers at Nanyang Technological University (Singapore) believe that they finally decipher the mystery why hot water freezes faster than cold water - a phenomenon that has been recorded since the time of the Greek philosopher Aristotle. Ancient Greece, but so far is a difficult puzzle for the scientific world.

The phenomenon of freezing hot water faster than cold water is also called "Mpemba effect" , named after a Tanzanian student. After attending cooking classes in the 1960s, student Erasto Mpemba discovered, the hot cream mixture would freeze faster than the cold ice cream mixture and ask this question to professors visiting his school in the year. 1968.

However, long before Mpemba was born, great philosophers in history such as Aristotle and Descartes also mentioned the phenomenon of strange frozen water.

Diagram describes "Mpemba effect": hot water will freeze faster than cold water.

Researchers have tried to explain the Mpemba Effect by various theories, including the notion that a warm container makes heat exposure to a refrigerator better, which helps heat transfer faster; or warm water evaporates faster, helps cool the water and makes the freezing process faster. However, no theory is accepted by the majority of scientific circles.

The Royal Society of British Chemistry even offered 1,000 tables to individuals or organizations to give a convincing solution.

In a new study, the team of scientists from Nanyang Technological University (Singapore), claimed, the Mpemba Effect was caused by the unique properties of the molecular bonds of water.

A water molecule is formed from covalent bonds between an oxygen atom and two hydrogen atoms. However, when a hydrogen atom of a molecule floats closer to the oxygen atom in another water molecule, they connect to each other, forming a hydrogen bond.

In cold water, the O - H covalent bonds are long, while the hydrogen bond is short;in the hot water is the opposite

Hydrogen bonds pull water molecules closer together, triggering natural repulsive forces between them, causing oxygen-hydrogen covalent bonds to stretch as well as store energy. Therefore, when the liquid is warm, it causes water molecules to be farther apart because the hydrogen bonds are stretched.

When water molecules shrink back and radiate energy, leading to cooling, it means that hot water freezes faster than cold water, helping explain the Mpemba Effect.

In order to prove their hypothesis, the team led by Dr. Xi Zhang calculated the extent of cooling, derived from molecular activity and conducted experiments to show that this was the culprit. The differences in freezing between hot and cold water have been observed.

However, some physicists note that the explanation currently cannot be used to predict the new properties of water, which can be generated by shortening covalent bonds. Therefore, there is still a step to be solved before the mystery is satisfactorily answered.