Why Hot Water Freezes Faster Than Cold Water

Why Hot Water Freezes Faster Than Cold Water

It’s Monday, and that might mean it’s counterintuitive science day. Up on the block: why does hot water freeze faster than cold water? It does, but it shouldn’t. Right? Wrong. The reason behind it is called the Mpemba effect, and a group of researchers think they’ve cracked the puzzle.

Now Xi and co say 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.

In fact, the effect is additional to the conventional process of cooling. So warm water ought to cool faster than cold water, they say. And that’s exactly what is observed in the Mpemba effect.