On the endless snowfield, when the extreme cold meets the hot water, the sky will bloom with brilliant ice fireworks, under the illumination of the sun, constituting a beautiful picture, which is the phenomenon of "splashing water into ice" of the beautiful southerners every year. When we are shocked by the "splashing water into ice" videos on major social platforms, we may have such doubts: Why splash hot water? Can hot water really freeze in the air?
Experienced friends will know that if we pour a scoop of cold water on a winter day, the scene that appears is often to pour ourselves into a soup chicken or splash someone else's head with water. This shows that hot water freezes faster than cold water in cold conditions. In fact, more than 2,000 years ago, Aristotle claimed to have observed this phenomenon. In the 1960s, a student named Erasto Mpemba in Tanzania was making ice cream, and in order to seize the limited space in the refrigerator, Mpemba did not wait for the milk to cool to room temperature like other students, but put the freshly boiled hot milk directly in the refrigerator. An hour and a half later, he found that his hot milk had frozen into ice cream, but the cold milk that had been put in the refrigerator with the hot milk was still in the state of thick milk milk. Confused, Mpemba asked his middle school physics teacher, only to be told, "You must be mistaken, that can't happen." ”
It wasn't until 1968 that the British physicist Denis Osborne came to Mpemba's high school to audit physics classes, and Mpemba immediately asked the professor questions. Osborn didn't believe it at first glance, but he didn't deny Mpemba either, but returned to the lab and immediately began experiments. Osborn then invited Mpemba to the University of Dares Salaam in Tanzania to study the phenomenon, which he named the "Mpemba effect." Mpemba and Osborn first demonstrated the Mpemba effect in 1969 in an article published in the journal Physics Education.
A diagram from a paper published in Physics Education by Mpemba and Osborn in 1969 provides evidence that hot water freezes faster than cold water
Curiously, however, they were unable to reliably replicate the initial results in subsequent experiments. This has led to a huge controversy: does the failure of the experiment mean that the Mpemba effect does not exist? Or is it because the experiments are too crude and don't take into account the effects of unknown variables?
In 2012, the Royal Society of Chemistry organised a competition with a prize fund of £1,000 to find the best explanation of the Mpemba effect. In the end, Nikolai Bregovich from the University of Zagreb won, describing his experiments in the winning article and limiting the scope of the analysis to the basic thermodynamic framework. He identified four factors that could be significant: evaporation, dissolved gases, convection, and supercooling. And he argues that supercooling is the most important factor, quoting Brownridge's article in his conclusion: "Hotter water freezes before colder water only when it is supercooled, and only when the nucleation temperature of colder water is a few degrees lower than that of hot water." Heating water may decrease, rise, or not change the natural freezing temperature"
In 2016, physicist Henry Burridge of Imperial College London and Paul Linden, a mathematician at the University of Cambridge, conducted an experiment that showed how sensitive this effect is to the details of the measurements. Unable to directly observe the freezing process, Burridge and Linden turned to measuring the time it takes for the water temperature to drop from its initial temperature to 0°C. They were surprised to find that this result depended on where the thermometer was placed in the water: if the thermometer was placed at the same depth, there would be no Mpemba effect between hot and cold water; But if the thermometer is placed at a depth of even 1 centimeter, it can be incorrectly "confirmed" by the Mpemba effect. They thus provided a detailed review of the confusion and lack of reproducibility of results from discussions around the topic since 1969 and published it in Scientific Reports, ruling out the supercooling phenomenon by measuring the time it takes for water to cool to 0°C. The title of the article sums up the problem well: "Questioning the Mpemba Effect: Hot Water Doesn't Cool Faster Than Cold Water." ”
So far, there is no satisfactory explanation for the Mpemba effect. But from Aristotle to Mpemba to Henry Burridge, over the past 2,000 years, it is through exploration and questioning that human beings have become more and more profound in their understanding of nature. And the reason why this mystery has not been completely solved is also because human beings still have a long way to understand nature, and they can only continue to move forward by seeking up and down.