The effect that causes hot water to instantly freeze in subzero temperatures is named after Ernesto Mpemba from the African Republic of Tanganyika. This natural phenomenon has been known to humanity since ancient times, mentioned by Aristotle and Descartes. Ernesto Mpemba and Denis Osborn published an article explaining this effect in the Physics and Education journal in 1969 – hence the exotic name.
According to the legend, young Mpemba thought he had made this discovery for the first time and pestered his schoolteacher to explain it. The teacher only laughed and kept saying "Oh, the Mpemba effect!". Mpemba then turned to a university professor, physicist Osborn, and the result became a popular science article.
Let us turn to a competent university scientist for commentary. Aleksandr Gerasimov, a physicist and senior researcher at the SUSU Sensory Systems Laboratory, explains the nature of this phenomenon:
"When hot water is sprayed in freezing temperatures, it evaporates much faster than cold water. The closer its temperature is to boiling point, the more intense the evaporation. And during evaporation, the liquid actively loses heat. Therefore, some of the tiny droplets have time to evaporate, while others instantly freeze, as they are rapidly supercooled. A similar principle works in reverse—for example, when ice melts: it takes heat from the surrounding water, and it can freeze again. These processes are interconnected. I think people have noticed this effect since ancient times, as soon as they learned to heat water in freezing temperatures; after all, it was probably often spilled, either accidentally or intentionally. The lower the temperature and the drier the air, the more pronounced the phenomenon is: in dry winter air, evaporation is easier. At -30°C or -40°C, the effect is already very noticeable. The spraying method is also important. The finer the droplets, the more visible the result: more fog and ice crystals are formed. If you pour water in large amounts, for example from a bucket, it will take extreme cold, around -50°C, to instantly freeze it. Therefore, it is better to spray it in small streams over a larger area—this way, the physics of the process is more clearly revealed."