Scientists have always pondered over why hot water froze faster than cold

A single water molecule consists of a relatively large oxygen atom joined to two smaller hydrogen atoms by standard covalent bonds.
But put water molecules together and hydrogen bonds also begin to play an important role. These occur when a hydrogen in one molecule comes close the oxygen in another and bonds to it.
Hydrogen bonds are weaker than covalent bonds but stronger than the van der Waals forces that geckos use to climb walls.
Xi's team said that hydrogen bonds also explain the Mpemba effect. Their key idea is that hydrogen bonds bring water molecules into close contact and when this happens the natural repulsion between the molecules causes the covalent 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. This allows the covalent molecules to shrink again and give up their energy. The important point is that this process in which the covalent bonds give up energy is equivalent to cooling.
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Source-ANI