Hot Water Turns to Ice Faster than Cold Water
Hot water can in fact freeze faster than cold water for a wide range of experimental conditions, and has been seen and studied in numerous experiments. Although this phenomenon has been known for centuries, and was described by Aristotle, Bacon, and Descartes, it was not introduced to the modern scientific community until 1969, by a Tanzanian high school pupil named Mpemba. Both the early scientific history of this effect, and the story of Mpemba’s rediscovery of it, are interesting in their own right — Mpemba’s story in particular providing a dramatic parable against making snap judgements about what is impossible.
Experimental Process
We start with two containers of water, which are identical in shape, and which hold identical amounts of water. The only difference between the two is that the water in one is at a higher (uniform) temperature than the water in the other. Now we cool both containers, using the exact same cooling process for each container. Under some conditions the initially warmer water will freeze first.
Mechanisms Underlying the Mpemba Effect
1. Evaporation
As the initially warmer water cools to the initial temperature of the initially cooler water, it may lose significant amounts of water to evaporation. The reduced mass will make it easier for the water to cool and freeze. Then the initially warmer water can freeze before the initially cooler water, but will make less ice. Theoretical calculations have shown that evaporation can explain the Mpemba effect if you assume that the water loses heat solely through evaporation. This explanation is solid, intuitive, and evaporation is undoubtedly important in most situations. But it is not the only mechanism. Evaporation cannot explain experiments that were done in closed containers, where no mass was lost to evaporation. And many scientists have claimed that evaporation alone is insufficient to explain their results.
2. Dissolved Gases
Hot water can hold less dissolved gas than cold water, and large amounts of gas escape upon boiling. So the initially warmer water may have less dissolved gas than the initially cooler water. It has been speculated that this changes the properties of the water in some way, perhaps making it easier to develop convection currents (and thus making it easier to cool), or decreasing the amount of heat required to freeze a unit mass of water, or changing the boiling point. There are some experiments that favour this explanation, but no supporting theoretical calculations.
3. Convection
As the water cools it will eventually develop convection currents and a non-uniform temperature distribution. At most temperatures, density decreases with increasing temperature, and so the surface of the water will be warmer than the bottom: this has been called a "hot top." Now if the water loses heat primarily through the surface, then water with a "hot top" will lose heat faster than we would expect based on its average temperature. When the initially warmer water has cooled to an average temperature the same as the initial temperature of the initially cooler water, it will have a "hot top", and thus its rate of cooling will be faster than the rate of cooling of the initially cooler water at the same average temperature. Got all that? You might want to read this paragraph again, paying careful distinction to the difference between initial temperature, average temperature, and temperature. While experiments have seen the "hot top", and related convection currents, it is unknown whether convection can by itself explain the Mpemba effect.
4. Surroundings
A final difference between the cooling of the two containers relates not to the water itself, but to the surrounding environment. The initially warmer water may change the environment around it in some complex fashion, and thus affect the cooling process. For example, if the container is sitting on a layer of frost which conducts heat poorly, the hot water may melt that layer of frost, and thus establish a better cooling system in the long run.
Extracted from:
https://math.ucr.edu/home/baez/physics/General/hot_water.html
