Why hot water is faster to form ice than cold water?

Water may be one of the most abundant compounds on Earth, but it is also one of more mysterious. For example, like most liquids it becomes denser as it cools. But unlike them, it reaches a state of maximum density at 4°C and then becomes less dense before it freezes.

In solid form, it is less dense still, which is why standard ice floats on water. That’s one reason why life on Earth has flourished— if ice were denser than water, lakes and oceans would freeze from the bottom up, almost certainly preventing the kind of chemistry that makes life possible.

The Mpemba effect is the observation that warm water freezes more quickly than cold water. The effect has been measured on many occasions with many explanations put forward. One idea is that warm containers make better thermal contact with a refrigerator and so conduct heat more efficiently. Hence the faster freezing. Another is that warm water evaporates rapidly and since this is an endothermic process, it cools the water making it freeze more quickly.

 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 vander Waals forces(V.D force) that geckos use to climb walls.

Hydrogen bonds also explain the Mpemba effect. The 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.