Under certain conditions, hot water may freeze before cold water, a phenomenon that depends on several factors, most notably evaporation, the movement of the liquid, and the timing of the formation of the first ice crystals.
According to the laws of physics, and intuitively, cold water should freeze before hot water because it needs to lose less heat. This is indeed what happens in most everyday situations. However, under certain conditions, hot water may turn to ice faster, a phenomenon known as the Mpemba effect
The phenomenon is named after Tanzanian student Erasto Mpemba, who observed in 1963, while preparing ice cream, that the hot mixture he placed in the freezer froze faster than the chilled mixtures prepared by his classmates. Later, physicist Dennis Osborn confirmed this observation, and he and Mpemba published their findings in 1969.
There is no single explanation for the Mpemba effect; it is likely that several physical factors interact simultaneously to produce this result, most notably:
Evaporation: Hot water evaporates at a faster rate, causing it to lose some of its mass. The less water remains, the less energy is needed to freeze it.
Convection: In hot water, strong convection currents form that transfer heat from the depths of the liquid to its surface and the walls of the vessel, thus accelerating the cooling process.
Contact with the freezer surface: The hot container may melt the thin layer of ice underneath it, thus coming into direct contact with the cold shelf, which enhances heat transfer compared to a container resting on top of a layer of ice.
Supercooling: Water doesn't always turn to ice once it reaches 0°C; it may cool below freezing and remain liquid for a period. In some cases, the first ice crystals may form in the hotter sample before the colder one, even though the latter is cooler.
According to experts, the result also depends on several factors, including the amount of water, its composition, the shape of the container, whether or not it has a lid, air movement, and even minute scratches on the container walls. It's also important to define what is meant by "freezing": is it the water reaching 0 degrees Celsius, the appearance of the first ice crystals, or complete freezing?
For this reason, two seemingly identical experiments may produce different results; in one, the cold water freezes first, while in the other, the hot water begins to freeze before its cold counterpart.
Generally, hot water can freeze faster than cold water, but only under specific conditions. The Mpemba effect does not mean that boiling water should always be put in the freezer, or that it always freezes faster than cold water.
More precisely, cold water freezes faster in most cases, but factors such as evaporation, convection, and the mechanism of ice crystal formation can sometimes give hot water an unexpected advantage.
