Why does warm water freeze faster than cold water in the so-called Mpemba effect? For more than fifty years scientists have sought to explain or debunk the phenomenon, but it is only in considering that range of hydrogen bond types in water that exist at different temperatures have US chemists finally found a plausible explanation. They demonstrate that in warmer water, there is an abundance of one particular type of hydrogen bond whereas in colder there is more of another class. The hot water hydrogen bonds are more electrostatic in character as opposed to the more "covalent" colder hydrogen bonds. As such, the hydrogen bond network is stronger in hotter water - yet another anomaly of this endlessly anomalous material. Given the stronger network sudden cooling can lock in this more "frozen" structure without having to dissipate quite so much energy as with the more fluid cold water hydrogen bond network.
Warming to new water theory