Cold Case Solved
Scientists Discover Why Hot Water Freezes Faster Than Cold Water
By: Ashley Peterson
For centuries, scientists have known that hot water freezes faster than cold water. Known as the Mpemba effect, the strange behavior of freezing water has been explored and recorded by scientists and great thinkers throughout history including Aristotle, Francis Bacon and Rene Descartes. But the reason why this strange phenomenon occurs has remained a mystery, until now.
Researcher Xi Zhang and a team of scientists from the Nanyang Technological University in Singapore believe they have finally ucovered the mystery of the Mpemba effect, suggesting the reason lies in the unique properties of the chemical bonds that hold water together.
Each molecule of water is made up of two hydrogen atoms that are covalently bonded to an oxygen atom. These bonds are well understood. By when a hydrogen atom in one water molecule drifts close to an oxygen atom in another water molecule, these molecules bond together to form what is called a hydrogen bond.
According to Xi Zhang, these hydrogen bonds cause the Mpemba effect: When water molecules come into close contact, a natural repulsion occurs between the molecules which causes the covalent bonds between oxygen and hydrogen to stretch and store energy.
As the liquid heats up, the hydrogen bonds stretch as the water gets less dense and the molecules move further apart. The additional stretch in the hydrogen bonds causes the covalent bonds to relax and shrink, giving up their energy. The process of covalent bonds giving up energy is the equivalent to cooling, so warm water should in theory cool faster than cold.
Although the research has not yet been peer-reviewed, the theoretical calculations suggest that the degree of the covalent bond relaxation precisely accounts for the experimental differences in the time it takes for hot and cold water to freeze.
“We demonstrate that the Mpemba paradox arises intrinsically from the release rate of energy initially stored in the covalent H-O part of the O:H-O bond in water albeit experimental conditions. Generally, heating raises the energy of a substance by lengthening and softening all bonds involved. However, the O:H nonbond in water follows actively the general rule of thermal expansion and drives the H-O covalent bond to relax oppositely in length and energy because of the inter-electron-electron pair coupling. Heating stores energy into the H-O bond by shortening and stiffening it. Cooling the water as the source in a refrigerator as a drain, the H-O bond releases its energy at a rate that depends exponentially on the initially storage of energy, and therefore, Mpemba effect happens.”
Did You Know?
The Mpemba effect was named after a Tanzanian student who took cooking classes in the 1960s and found that hot ice cream mixture froze faster than a cold one.
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