Mystery of why hot water freezes faster than cold is solved - and it's all down to the strange behaviour of atom bonds



The Mpemba effect, named after Tanzanian student Erasto Mpemba, is the theory that warmer water can freeze faster than colder water

Research by Nanyang Technological University in Singapore has revealed the bonds that hold water molecules together cause the effect



Scientists have been trying to explain why water behaves in the strange way it does since ancient times

Scientists have known that hot water freezes faster than cold water, since the time of Aristotle in Ancient Greece.

However, until now they have struggled to explain why.

A team of scientists in Singapore believe they have finally revealed the mystery of the Mpemba effect and it lies in the unique properties of the bonds that hold water together.

This graph shows how the Mpemba effect - how hot water freezes faster than cold. It shows how rapidly boiling water cools compared to the water that starts off at a lower temperature. Scientists think they have finally solved the mystery and believe the secret lies in the unique properties of the bonds that hold water together

WHAT IS THE MPEMBA EFFECT?

The Mpemba effect is the theory that warmer water can freeze faster than colder water. The effect has been noted since ancient times but scientists have struggled to explain why it occurs. A team of Singapore scientists finally think they have solved the mystery and think the secret lies in the unique properties of the bonds that hold water together. Hydrogen bonds bring individual water molecules into close contact, which triggers natural repulsion between the water molecules and causes the bonds between oxygen and hydrogen atoms to stretch as well as store energy. So as the liquid warms, it makes the water molecules sit further apart from each other as the hydrogen bonds stretch. When the molecules shrink again and give up their energy, this results in it cooling, which the scientists say means that warm water cools faster than cold water and explains the Mpemba effect.

The Mpemba effect - why hot water freezes more quickly than cold water - is named after a Tanzanian student who took cookery classes in the 1960s and found a hot ice cream mixture froze faster than a cold one.

But the strange behaviour of freezing water has been noted by great thinkers throughout history, including Aristotle and Descartes.

Scientists have tried to explain the Mpemba effect before and theories included the idea that warm containers made better thermal contact with frezers to conduct heat faster and that as warm water evaporates more swiftly, it cools the water, which allows it to freeze faster.

But a study led by Xi Zhang at the Nanyang Technological University in Singapore, said the reason for the Mpemba effect is down to the unique properties of the molecular bonds that hold water together, Physics blog Medium.com reported.

A single water molecule is made from a large oxygen atom joined to two hydrogen atoms with covalent bonds (a chemical bond that involves the sharing of two electron pairs between atoms).

But when a hydrogen atom in one molecule drifts close to an oxygen atom in another water molecule, it bonds with it, creating what is called a hydrogen bond.



It is these hydrogen bonds that behave in a peculiar way and have attracted the attention of the researchers.

A water molecule is made from an oxygen atom joined to two hydrogen atoms with covalent bonds (a bond that involves the sharing of two electron pairs). When a hydrogen atom drifts close to an oxygen atom, it bonds with it, creating a hydrogen bond. It's the strange behaviour of these bonds that explains the Mpemba effect

The strange behaviour of hot water freezing more quickly has been noted by great thinkers throughout history, including Aristotle (pictured) and Descartes

Strangely, while hydrogen bonds are generally weaker than covalent bonds, they are stronger than the 'van der Waals' force' that is the sum of the attractive forces between molecules other than those due to covalent bonds.



Chemists have long suspected the hydrogen bonds were what gives water some strange properties and enables its boiling point to be much higher than other liquids composed of similar molecules - because the hydrogen bonds hold it together so well.

Research into the exact roles the hydrogen bonds play have been investigated by scientists who have recently found water molecules restricted into tiny tubes form chains and are linked together by hydrogen bonds.

It is these tiny chains that enable a plant to pull water molecules up through its roots.

But now Dr Zhang's team believe these bonds explain the Mpemba effect, as the hydrogen bonds bring individual water molecules into close contact, which triggers natural repulsion between the molecules and causes the covalent bonds between oxygen and hydrogen bonds to stretch as well as store energy.

So as the liquid warms, it makes the water molecules sit further apart from each other as the hydrogen bonds stretch.

The Mpemba effect is named after a Tanzanian student who took cookery classes in the 1960s and found a hot ice cream mixture froze faster than a cold one.

When the molecules shrink again and give up their energy, this results in it cooling, which the scientists say means that warm water cools faster than cold water and explains the Mpemba effect.

To prove their theory, the chemists calculated the size of the extra cooling caused by the molecular activity and showed that it accounts for the observed differences between the freezing of hot and cold water in experiments.