In 1963, Tanganyik schoolboy Erasto Mpemba approached a physics teacher in class and asked him to explain why hot water freezes faster than cold water, but he only laughed at the student, saying: "This is not world physics, but the physics of Mpemba." But the boy stood his ground, because he saw it with his own eyes putting two mugs of water in the refrigerator, and the hot one froze faster.
Mpemba asked the same question to Dennis Osborne, a physics professor, who came to school. The carried out experimental verification confirmed the existence of the effect, but did not provide an explanation. In 1969, the journal Physics Education published a joint article by Mpemba and Osborne describing the effect. In the same year, George Kell of the Canadian National Research Council published an article describing the phenomenon.
There are several ways to explain this paradox:
Hot water evaporates faster from the container, thereby reducing its volume, and a smaller volume of water with the same temperature freezes faster. Cold water should freeze faster in sealed containers.
The presence of a snow lining in the freezer compartment of the refrigerator. The hot water container melts the snow beneath it, thereby improving thermal contact with the freezer wall. The cold water container does not melt snow underneath. If there is no snow lining, the cold water container should freeze faster.
Cold water begins to freeze from above, thereby worsening the processes of heat radiation and convection, and hence the loss of heat, while hot water begins to freeze from below. With additional mechanical stirring of water in containers, cold water should freeze faster.
The presence of crystallization centers in cooled water - substances dissolved in it. With a small number of such centers, the transformation of water into ice is difficult and even its supercooling is possible when it remains in a liquid state, having a subzero temperature. With the same composition and concentration of solutions, cold water should freeze faster.
Due to the difference in energy stored in hydrogen bonds. The warmer the water, the greater the distance between the liquid molecules is due to the increase in repulsive forces. As a result, hydrogen bonds are stretched, and therefore store more energy. This energy is released when the water cools - the molecules move closer to each other. And the release of energy, as you know, means cooling.
But an unambiguous answer to the question of which of them provides one hundred percent reproduction of the Mpemba effect has not been received.