[Mpemba Effect] Why Hot Water can Freeze Faster than Cold?

carlos-cabeza (68)in #science • 8 years ago

Erasto B. Mpemba was a student of an institute of Tanzania that in 1963 was making ice cream in the classes of kitchen. Freezing the different mixtures discovered that the one that was hot was frozen before the cold one. When he asked his teacher about this phenomenon, he respectfully laughed in his face saying: "All I can say is that this is physical Mpemba, and not universal physics."

Physicists have proposed several theories with several possible explanations, including rapid evaporation, reducing the volume of water, a layer of ice insulating ice water, and different concentrations of solutes. But the answer has been very difficult to define because it is not a reliable effect.

To understand why, you have to understand the main causes:

In the hot vessel the liquid flows better, whereby the hot water from the central zone moves more rapidly towards the walls of the vessel or toward the upper surface, causing it to cool.
The water at higher temperature evaporates more.
The hotter a liquid, the less dissolved gases you have left (gases make it difficult to freeze).

The explanation of the phenomenon is not unique; there are several factors that influence this fact. The first is that hot water has less dissolved gases than cold water. Gases make it difficult to freeze water. Another factor is that the degree of evaporation of hot water is greater. And a third factor is convection, the movement that occurs in the hot water that facilitates that the temperature is distributed in a more uniform way and the transfer of heat to the outside: in the case of the cold water would begin to form a layer of ice in The top of the container which would make it difficult to freeze the bottom layer.

There are other answers… The hot water cools faster because of a curious reason: the hotter it is, the less gas bubbles it contains. And what does this have to do with freezing? For the bubbles act as "link-up" so that the water molecules begin to orient and form the crystalline structure of the ice. The less "link-up" the water has, the easier it is to keep it liquid below the freezing point.

At the Southern Methodist Universities in Dallas and the University of Nanjing in China, have conducted a series of studies on the subject of molecular union that we talked about earlier and they say:

"Although the experience has been repeated many times by different research professors, there is nothing concrete about what is the most adequate explanation".