Why hot water freezes faster than cold water

Telegraph : Named the Mpemba effect after the author of a 1969 paper on the subject, the observation that hot water freezes faster than cold water has puzzled scientists since at least the time of Aristotle. Now, Sun Changqing and Xi Zhang of Nanyang Technological University in Singapore propose that the explanation may have to do with the hydrogen bonds that bind water molecules to one another—the source of water’s surface tension. Changqing and Zhang say that the rate of energy release when water is placed into a freezer depends on the initial state of the water. As water is heated, the hydrogen bonds between molecules may cause the covalent bonds between the hydrogen and oxygen atoms to shorten and store more energy. As the water is cooled, the bonds relax and release energy at a rate exponentially dependent upon the amount of energy stored, so that hot water releases energy faster than cold water and therefore freezes more quickly. The effect may be more complex than that, however, as previous work has suggested that convection currents may be involved as well.