Aristotle was the first man who discovered that hot water freezes faster than cold water. Or he was the first to record it, at least. Two millennia later, in 1963 Tanzanian student observed the same phenomenon and since then it was named after him – the Mpemba effect.
[Image Source: Nuclear Darkness]
There were several theories trying to explain the Mpemba effect. One of them is that a frost layer on top insulates the water below and slows freezing; another is that hot water evaporates faster which lowers the volume available for freezing. Also a theory including different substances dissolved into water was proposed, but none of them offered satisfying explanation of the phenomenon.
Xi Zhang leads a team of scientists at the Nanyang Technological University in Singapore that proposes new, promising theory for the Mpemba effect. It is known that two hydrogen atoms and a single oxygen atom form one water molecule via covalent bonds. Water molecules are connected with each other by much weaker hydrogen bonds. These hydrogen bonds are formed when a hydrogen atom from one molecule arrives close to an oxygen atom from the other one.
Xi Zhang's team suggestion is that when the water is heated the hydrogen bonds are stretched while the water density lowers and molecules move away from each other. Along with that, covalent bonds shrink at some scale, which leads to releasing some amount of bonds energy. The process of such releasing of energy is essentially the same as cooling, so this is why warm water gets frost faster than cold water.