Science
This sounds like one of the questions that even a child should know the answer to, but in fact, even scientists can't answer it. Why does hot water sometimes freeze faster than cold water? The Royal Society of Chemistry is offering a £1,000 prize to anyone who can answer this question.
This phenomenon, called the Mpemba effect, seems scientific nonsense at first glance. How can hot water freeze faster than cold water? Hot water must lose more heat than cold water to freeze, so why should this happen faster?
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However, this paradox was confirmed back in ancient times. It is mentioned by Aristotle, as well as by the fathers of modern science, Francis Bacon and René Descartes in the 17th century. This effect is named after Erasto Mpemba, a student from Tanzania, who experimented with making ice cream from milk in 1963. Students had to heat the milk and let it cool, then place it in the freezer to freeze. But the boy was worried that he would not have enough space in the freezer and placed the milk when it was still hot and it froze faster than the others.
Then, several years later, Mpemba found out that this contradicted Isaac Newton's theory of heat transfer. He asked his teacher who asked the lab technician to do the experiment again, and the results were soon published in an educational journal.
However, no one really knows if the Mpebma effect actually exists. You think it's easy to test, but it's not. Does this mean the time when the ice starts to appear or when all the water is frozen?
Both of these phenomena are difficult to observe. And there are so many things that can vary: the amount of water, the shape of the container, the initial temperature difference, the degree of cooling, etc. You use running or distilled water, water deprived of air, filtered water?
Freezing is generally a rather capricious process: it can be caused by tiny scratches on the walls of the bulb, or by suspended dust in the liquid, and therefore it is almost impossible to do it really identical samples that differ only in their initial temperature. For this reason, even two samples of the same temperature usually freeze at different times.
This is one common explanation for the Mpebma effect. Hot water should evaporate faster, so there might have been less liquid to freeze, and it happened faster. Small gas bubbles could also act as a source of ice crystal formation, and hot water retains less dissolved gas than cold water.
All this suggests that one experiment won't tell you much, and you need to do quite a few experiments under different conditions to figure out what's important and what's not. So, everyone has the opportunity to show scientifically convincing answers by the end of July in order to win the prize.