Ever came across the Mpemba effect? The property of hot water which makes it freeze faster than cold water is what we are talking about. Scientists the world over have known the phenomenon, but haven’t yet been able to zero in on why such a thing happens and how! Research has been on for decades, and now finally a scientist has come out with theories on the Mpemba effect. Ever since Aristotle’s era, many scientists have noted this phenomenon and have been researching on it.
As per new findings, the Mpemba effect could be explained via a clutch of new theories. One of them is that freeing point for hot water is made possible due to the faster evaporation of hot water, which reduces the volume left to freeze. It has also been explained that formation of a frost layer on cold water insulates it. Another theory is that different concentrations of solutes such as carbon dioxide are driven off when the water is heated making temperature drop.
According to James Brownridge, radiation safety officer with the State University of New York, he has evidence that supercooling is involved. Brownridge has been studying the effect in his spare time for the last decade and have churned out hundreds of experiments to this effect. Brownridge, according to physorg.com, has found water usually supercools at 0°C and only begins freezing below this temperature. The freezing point is governed by impurities in the water that seed ice crystal formation. Impurities such as dust, bacteria, and dissolved salts all have a characteristic nucleation temperature, and when several are present the freezing point is determined by the one with the highest nucleation temperature, he adds.
Brownridge went about his experiments by taking two water samples at the same temperature. After having placed them under sub-zero conditions, he found that one would usually freeze before the other, owing to a slightly different mix of impurities.
Brownridge removed the samples from the freezer, warmed one to room temperature and the other to 80°C and then froze them again. The results were that if the difference in freezing point was at least 5°C, the one with the highest freezing point always froze before the other if it was heated to 80°C and then re-frozen, says physorg.
The study showed that the hot water cools faster because of the bigger difference in temperature between the water and the freezer, and this helps it reach its freezing point before the cold water reaches its natural freezing point, which is at least 5°C lower. The result also banks on conditions such as the location of the samples in the freezer, and the type of container.
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