According to a curious phenomenon called Mpemba effect, hot water is supposed to freeze faster than a cooler sample. It was named after a Tanzanian student who first observed this process and then published a paper in 1969. But he was by no means the first to observe it. Famous scientists like Aristotle, Bacon and Descartes also recorded it in their books. But what is the reason behind this strange property of water? We all know water has another peculiar thermal property as it expands in the last 4 degrees leading to freezing. Hydrogen bonded spacing is the reason behind that. What is the reason attributed to the faster freezing? Here we attempt to answer that question based on research.
Various possibilities have been suggested by researchers regarding the Mpemba effect, often diverging with each other. It was claimed that the hot water makes evaporation faster. Thus, a lesser volume is left to freeze. Then it was also said that different concentrations of gases and impurities were filtered when the sample of water was heated though it remained in the cooler samples. But all these theories are not known for their authenticity, as the Mpemba effect is not always there as sometimes, the cold water does freeze faster than hot. The effect has never been answered concretely.
This time another attempt to respond to the elusive question has been made at the Nanyang Technological University in Singapore. Led by Xi Zhang, the researchers have come to the conclusion that the nature of the chemical bonding is the reason behind this different behaviour of water. There are two kinds of bonding prevalent in water molecules. One is the covalent bond that is between directly bonded hydrogen and oxygen atoms. Then there is the hydrogen bond in which the polarized hydrogen and oxygen atoms in nearby water molecules experience 20% forces equivalent to that of a covalent bond.
When the water is warmed, and the volume increases, the polarized hydrogen and oxygen move into more natural positions and thus, there is no possible repulsion between the same charges. When the water is already cold, repulsive forces are dominant, and they cause the liquid to cool more with more difficulty. The study also suggests that the difference between covalent bond relaxation is the explanation behind varying experimental differences.
The study, though recent still fails to give sturdy answers. Since water is a universal solvent, the approach regarding the ionized particles may have had more weight behind it. Anyway, a new angle does help in getting the eventual reality out.