Despite sounding like the most egregious contradiction in physics, hot water appears to freeze faster than cold water under certain circumstances.
after centuries of experiments demonstrating this phenomenon, no one’s been able to explain it.
Now physicists are pointing to strange properties of hydrogen bonds as the solution to one of the oldest mysteries in physics – but others are claiming the so-called Mpemba effect doesn’t even exist at all.
What is Mpemba Effect
Hot water seems to freeze faster than cold water, known as the Mpemba effect. The effect was named after the Tanzanian student who in 1963 noticed that hot ice cream mix freezes faster than a cold one. The effect was first observed by Aristotle in the 4th century BC, then later Francis Bacon and René Descartes.
Theories for the Mpemba effect have included faster evaporation of hot water, therefore reducing the volume left to freeze; formation of a frost layer on cold water, insulating it; and different concentrations of solutes such as carbon dioxide, which is driven off when the water is heated. Unfortunately the effect doesn’t always appear – cold water often does actually freeze faster than hot, as you would expect. But this Mpemba effect occurs regularly, and no one has ever been able to definitively answer why.
Why Does Hot water freeze faster than cold
The most commonly proposed hypothesis – and one that’s probably somewhat responsible for the effect – is that hot water evaporates more quickly, losing mass and therefore needing to lose less heat in order to freeze. However, scientists have also demonstrated the Mpemba effect with closed containers where evaporation doesn’t take place.
Another theoretical speculation is that water develops convection currents and temperature gradients as it cools – a rapidly cooling glass of hot water will have greater temperature differences throughout, and lose heat more quickly from the surface, whereas a uniformly cool glass of water has less of a temperature difference, and there’s less convection to accelerate the process. But this idea has not been entirely verified either.
The video below shows what is really happening when you expose hot water on – temperature
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