Why does hot water sometimes freeze faster than cold water?

Hot water can freeze faster than cold water under certain conditions; this phenomenon is known as the Mpemba effect.

What the Mpemba effect is

The Mpemba effect describes the observation that, in some experiments, a sample of water at a high temperature reaches the solid state before a sample at a lower temperature, provided the same freezing conditions are applied.

Factors that favor the phenomenon

• Evaporation: Hot water loses mass more quickly, which reduces the volume that needs to freeze.
• Convection: Convective currents in hot water promote a more uniform temperature distribution.
• Dissolved gases: Hot water retains less dissolved gas, which reduces bubble nucleation and speeds up ice formation.
• Supercooling: Cold water tends to supercool more before freezing, while hot water usually begins crystallization sooner.

Experimental conditions

The effect is not always observed; it depends on the purity of the water, the shape of the container, the ambient temperature, and how the container is placed in the freezer.

In practice, to reproduce the effect it is recommended to:

• Use containers made of similar material for both samples.
• Place the container in the coldest part of the freezer.
• Avoid sudden movements that disrupt convection.

In summary, the speed at which hot water freezes is not an absolute rule, but the Mpemba effect shows that the initial temperature influences freezing dynamics when the described processes come into play.