Water is a vital resource for the life of all living beings that inhabit the planet, but the truth is that we still have much to discover about it because its behavior still has some unknowns. One of these is the Mpemba effect, one of the water phenomena known since ancient times, as it was discovered by Aristotle almost 2,000 years ago when he stated that hot water froze faster than cold water, but it did not receive a name until 1963.
The Mpemba effect has become famous thanks to videos of people living in places with extremely cold temperatures showing how boiling water freezes instantly when thrown outdoors. It is certainly beautiful to watch, although the explanations are not yet entirely clear.
Discovery of the Mpemba effect
Aristotle observed how the people of present-day Turkey sprinkled the stakes of their palisades with hot water to secure them because in this way they froze earlier. But he was not the only one to notice it before it received a name, Francis Bacon and René Descartes also noticed this curious phenomenon. But it was not until 1963 that modern science became aware of it and gave it a name of its own.
This phenomenon is named after the Tanzanian scientist Erasto Bartolomeo Mpemba, who while in high school noticed that the ice cream mixture he had prepared and put in the freezer when it was still warm, was freezing faster than that of his classmates who had let it cool.
Some years later, he published an article together with the British scientist Denis Osborne in 1969, in which they explained in detail how this effect works. As part of their research, they conducted a study on the freezing time of different samples of water inside a freezer, resulting in the freezing time being longer when the water was near 25ºC and shorter when it was near 90ºC.
It should be emphasized that this phenomenon occurs under quite specific circumstances where large temperatures are required for it to take effect. For example, water at 5°C will freeze before watering at 35°C but water at 70°C will take longer to freeze than water at 90°C.
More recent studies have paid close attention to the temperature variables, as well as the number of dissolved substances in it, as these have not been able to match the conclusions that Mpemba and Osborne presented. In 2012, the Royal Society of Chemistry offered a prize of £1000 to whoever could come up with the best explanation for such a phenomenon, unfortunately of the 22,000 papers submitted none was convincing enough.
What gives rise to the Mpemba effect
The effect is more complex than it seems, requiring specific variables and contexts. The first variable is the evaporation of water when it is hot because as it evaporates faster it loses mass and therefore will need to lose less heat to freeze. However, this is refutable because the effect also occurs in closed containers where evaporation is not possible.
Another possible explanation is convection currents within the liquid itself, where when containers are placed in the freezer, their freezing does not take place uniformly but starts at the outer surface and creates a temperature gradient that accelerates the cooling of the water. However, cold water may have a larger amount of dissolved gases that reduce the thermal conductivity of the water, so the cooling of areas further from the surface is cooled for a greater amount of time.
Simulations of clusters of water molecules have revealed that the strength of hydrogen bonds in a water molecule depends on the arrangement of neighboring water molecules. It is because of this that as water is heated, the weaker bonds break and form large molecules that make way for ice, the first step in freezing.
For cold water to rearrange itself in this way, the weak hydrogen bonds must first be broken. Although some explanations can support how the Mpemba effect is generated, more studies are still needed to understand with certainty what causes boiling water to freeze instantly in an environment of extremely cold temperatures. It is undoubtedly one of the most beautiful water phenomena to observe.
Story originally published in Spanish in Ecoosfera
