Internet users from Chicago to Siberia post photos and videos of splashed water turning into ice before reaching the ground. They call it the #ubak challenge. Boiling water is used for the experiment, not cold water. Want to know why?
The Mpemba paradox
In the 60s of the last century, a student from Tanganyika, Erasto Mpemba, noticed that heated sweet liquid turns into ice cream much faster than cold one. These observations gave him the idea that hot water will be the same, which is contrary to logic. After all, the "path to freezing" for boiling water should be longer than for chilled liquid.
With this question, the boy turned to Professor Dennis Osborne, who once visited the school. Erasto puzzled him. At the direction of the physicist, experiments were carried out at the University of Dar es Salaam, which proved that the young researcher was telling the truth. Hot water did freeze faster than cold water in some cases! This phenomenon has received the name of the Mpemba paradox, an unambiguous explanation for which has not yet been found. It is surprising that the paradox was described by Aristotle - the inhabitants of Pontus made ice from hot water. And that was more than 2,000 years ago.
This is how the Mpemba effect looks in Chicago. There the challenge is called the Boiling Water Challenge.
In 2012, the Royal Society of Chemistry of Great Britain announced an international competition for the best substantiation of the Mpemba effect. And that's what came of it.
Promotional video:
What explains the paradox?
22,000 scientific papers were sent to the international competition, but none of them was recognized as the only correct one. According to one version, hot water in the open air freezes faster due to evaporation. During evaporation, the volume decreases, and the smaller volume solidifies much faster.
On the other hand, the solution lies in the content of dissolved gases. When heated, they evaporate and no longer interfere with the extreme cooling of the boiling water. A similar effect is observed with different content of solutes, if water is taken from different sources. More impurities make freezing difficult. But under the experimental conditions, the water samples must be identical in composition.
And this is how the residents of Novosibirsk have fun. Taking this opportunity, I say hello to my friend, who moved there, but did not like the Siberian frosts)))
The Mpemba paradox can also arise due to the rate at which energy is released from hydrogen bonds between water molecules. When heated, the internal energy of a substance increases, and the hydrogen bonds are lengthened. When cooled, the water molecules come closer to each other again, releasing the stored energy. There is definitely more boiling water in stretched hydrogen bonds than in chilled water.
Despite the fact that all hypotheses seem reasonable, scientific controversy about the Mpemba paradox continues. It has already been noted that it also has the opposite effect, which is observed when heating cold water. Research continues and, perhaps, soon there will be one less natural mystery in the world. But how many incredible photos she gave us!