Although it sounded pointless, did you know that hot water actually freezes faster than cold water? It seems impossible, right? But look at the video that this Canadian recorded, throwing boiling water up when the outdoor temperature was at -30 ° C.

Amazing how the water turns snow so fast, do not you think? But how does this happen?

The Mpemba Effect

The rapid freezing of hot water is known as Mpemba Effect. Known in ancient Greece, in fact the phenomenon was only recognized in the 60’s. At the time, a 13-year-old student, named Erasto Mpemba, told his science teacher that he could make ice cream faster when he put the mixture still warm in the freezer.

And although the phenomenon was discovered in 1969, scientists have so far been unable to explain the reason conclusively.

How does it happen?

In fact, the effect is not constant and there are cases where cold water can also freeze quickly. In the experiment which describes the “Mpemba effect”, the liquid at ambient temperature reaches before the solidification temperature (ie zero degree Celsius), but does not freeze immediately. The hottest liquid turns ice as soon as it reaches the same temperature.

The explanation lies in the fact that water cooling is not linear and through experiments we have identified the main factors that contribute to this achievement. They are good contact and conduction, convection, evaporation and dissolved gases. Each of these factors puts the hot water to the advantage of the water that is at ambient temperature, allowing it to freeze first.

When you place water at ambient temperature for freezing, you will note that after a certain time a thin layer of ice will form on the upper surface and on the sides. This layer of ice becomes a thermal insulation and prevents the conduction of heat between the water and the base of the container that is in contact with the refrigerator, which delays the cooling process.

However, if the water is previously heated, it will be found that these layers take longer to emerge, due to the convection movement carried out by it. When the surface water begins to freeze it will immediately move to the bottom and the hot water that was in the bottom will rise to the surface. This is because as the temperature of the surface water drops its density increases. Soon it will tend to descend and in its place will rise the hot water with less density. This convection current will allow a homogeneity of the temperature of the water during its cooling, avoiding the rapid appearance of the thin insulating layers of ice.

Because hot water evaporates more easily than water at ambient temperature, it is believed that after placing in the refrigerator to freeze the hot water would be to advantage because its mass would slightly reduce, while its temperature would decrease faster. That is, this surface evaporation would contribute to the efficient heat exchange and reduction of the mass of water to be frozen. However, this loss of mass is not as significant and even in experiments with closed containers the effect was verified.

It is known that in the water there is the presence of dissolved gases, such as: oxygen gas, carbon dioxide, in addition to other impurities. The higher the amount of these dissolved gases the lower the temperature to reach for solidification of the water, in other words, the dissolved gases make it difficult for the water to reach the melting point.

The water that is preheated partially expels the gases that are in it, because the solubility of these is lower at high temperatures and when this same water begins to cool it will have less dissolved gases than the water that has not been heated, thus its melting point will be increased and it will freeze first.

So, did you like to know that? Keep an eye on our blog for more curiosities.