The Mpemba effect, named after Erasto Mpemba, is the assertion that, in some circumstances, warmer water can freeze faster than colder water. Although there is anecdotal support for the effect, there is no agreement on exactly what the effect is and under what circumstances it occurs. There have been reports of similar phenomena since ancient times, although with insufficient detail for the claims to be replicated.
The phenomenon seems contrary to intuition, but a number of possible explanations for the effect have been proposed. Further investigations will need to decide on a precise definition of “freezing” and control a vast number of starting parameters in order to confirm or explain the effect.
The behaviour seems contrary to natural expectation but many explanations have been proposed to explain the claimed effect.
- Evaporation: The evaporation of the warmer water reduces the mass of the water to be frozen. Evaporation is endothermic, meaning that the water mass is cooled by vapor carrying away the heat, but this alone probably does not account for the entirety of the effect.
- Convection: Accelerating heat transfers. Reduction of water density below 4 °C (39 °F) tends to suppress the convection currents that cool the lower part of the liquid mass; the lower density of hot water would reduce this effect, perhaps sustaining the more rapid initial cooling. Higher convection in the warmer water may also spread ice crystals around faster.
- Frost: Has insulating effects. The lower temperature water will tend to freeze from the top, reducing further heat loss by radiation and air convection, while the warmer water will tend to freeze from the bottom and sides because of water convection. This is disputed as there are experiments that account for this factor.
- Supercooling: It is hypothesised that cold water, when placed in a freezing environment, supercools more than hot water in the same environment, thus solidifying slower than hot water. However, super-cooling tends to be less significant where there are particles that act as nuclei for ice crystals, thus precipitating rapid freezing.
- Solutes: The effects of calcium carbonate, magnesium carbonate among others.
- Thermal conductivity: The container of hotter liquid may melt through a layer of frost that is acting as an insulator under the container (frost is an insulator, as mentioned above), allowing the container to come into direct contact with a much colder lower layer that the frost formed on (ice, refrigeration coils, etc.) The container now rests on a much colder surface (or one better at removing heat, such as refrigeration coils) than the originally colder water, and so cools far faster from this point on.
- The effect of heating on dissolved gases; however, this was accounted for in the original article by using boiled water.