The phenomenon that warmer water freezes faster than colder water has baffled scientists since ancient times. This paradoxical observation is known as the Mpemba effect, after a Tanzanian student who, much to his surprise, found that hot ice cream mixture cooled faster than cold ice cream mixture. After centuries of inadequate explanations, a team of scientists, led by Xi Zhang at the Nanyang Technological University in Singapore, claim they have uncovered this ancient mystery.
If this is your first exposure to the Mpemba effect, I can only imagine that you’re feeling more than a little skeptical. This phenomenon can seem very counterintuitive. It is important to note that the Mpemba effect is only observed under specific initial temperatures, container shapes and cooling conditions. A study carried out by Dr. Auerbach of Michigan University showed that the Mpemba effect was most likely to occur at cooling temperatures between -6°C and -12°C.
Zhang and his colleagues have found evidence which suggests that the Mpemba effect can be attributed to the unique properties of the hydrogen bonds that hold water molecules together.
They claim that when water molecules are brought close in contact with one another by hydrogen bonds, the covalent O-H bonds of water become compressed and store energy. As the water heats up, the hydrogen bonds relax and the water molecules move apart. This allows O-H bonds to relax and give up energy.
Cooling occurs when thermal energy is lost to the surroundings. The additional energy lost through the relaxation of O-H bonds causes warmer water to cool faster than colder water.
Hydrogen Bonds : Courtesy www.flickr.com
Researchers calculated the additional thermal energy lost by covalent bond relaxation and found it to exactly account for the different cooling times of the warmer and cooler water.
Although this explanation is very convincing, Zhang and his colleagues have yet to use this new theory to explain another property of water. So, while the mystery of the Mpemba effect may have been solved, more work is needed before this theory can be fully accepted.