Does hot water freeze faster than cold water? It seems obvious that the answer should be no, because hot water takes longer to cool down, and so it couldn’t possible freeze faster. But hot water seems to freeze faster than cold water, this effect is known as the Mpemba effect. The effect was named after the Tanzanian student who in 1963 noticed that hot ice cream mix freezes faster than a cold one.
Now a team of physicists from the Nanyang Technological University in Singapore, led by Xi Zhang, have found evidence that it is due to the unique properties of the different bonds that hold water together that provide this effect.
Let’s get to know what’s so odd about the bonds in water? Each water molecule consists of a relatively large oxygen atom joined to two smaller hydrogen atoms by standard covalent bonds. But when you put water molecules together, the separate water molecules are also bound together by weaker forces generated by hydrogen bonds. These forces occur when a hydrogen atom from one molecule of water sits close to an oxygen atom from another.
The team suggests that these are the bonds that cause the Mpemba effect. They propose that when the water molecules are brought into close contact, a natural repulsion between the molecules causes the covalent bonds O-H bonds to stretch and store energy. But as the liquid warms up, it forces the hydrogen bonds to stretch and the water molecules sit further apart. The stretching in the hydrogen bonds allows the covalent bonds to relax and shrink somewhat, which causes them to give up their energy. The process of covalent bonds giving up their energy is essentially the same as cooling, and so warm water should in theory cool faster than cold. And that’s exactly what is observed in the Mpemba effect. The team’s calculations suggest that the magnitude of the covalent bond relaxation accounts for the experimental differences in the time it takes for hot and cold water to freeze.
So while these guys may well have solved the riddle of Mpemba effect, they will probably need to work a little harder to convince everyone. Nevertheless, interesting stuff!