why does hot water freeze faster than cold?


The Mpemba effect: Under the right circumstances, hot water can freeze faster than cold. This mystery has intrigued thinkers since Aristotle, but a team of Spanish physicists have discovered how and why this apparent paradox can occur.

The answer, as described in a 2017 article in Physical examination letters by Antonio Lasanta of Charles III University in Madrid and colleagues, depends on the speed of individual water particles as they rush in all directions like ants in a nest.

Why is the Mpemba effect so named?

Although the Mpemba effect was noted by many scientists and nature philosophers over the centuries Рincluding Ren̩ Descartes and Francis Bacon Рit was the subject of little modern study until the 1960s. things changed when a Tanzanian teenager named Erasto Mpemba noticed that an ice cream mixture that had been heated froze more easily than a mixture that was cold. Mpemba asked a physicist who was visiting his high school about it, and together they confirmed the existence of the effect in the lab.

Since then, various explanations have been proposed for the Mpemba effect: that the evaporation of hot water dissipates heat more efficiently, or unusual flows in the fluid, or even supercooling, in which the temperature of the water drops well below zero before turning to ice.

None of the explanations were entirely convincing. Indeed, a 2016 study concluded “somewhat sadly” that he could not find any evidence that the effect even exists.


Read more: Ice, ice maybe?


In order to study the problem, the Spanish researchers began by defining it more clearly. Imagine two beakers of water, one hotter and one colder, placed in a freezer. If the Mpemba effect applies, the hotter will reach zero degrees earlier than the colder.

Inside each beaker, the molecules that make up water swarm in all directions. If the water heats up, they move faster; if it cools down, they slow down by crawling; and if it freezes, they get stuck, squirming weakly in place.

The Lasanta team analyzed a simplified version of this situation, in which the particles in the liquid are tiny spheres that lose a tiny bit of energy each time they collide with each other.

Conventional wisdom is that the time it takes for each beaker of water to freeze depends only on its initial temperature. Particles in warmer water move faster, which means they have more slowing down – so the hotter the liquid, the longer it should take.

The researchers found, however, that temperature was not the only important factor.

If the water particles are like ants running around a nest, the temperature of the whole fluid is their average speed. So a nest where all the ants are walking at 50 yards an hour looks like a nest where half is 50, a quarter sprints at 75 and the last quarter drags at 25.

However, the number of outliers – lazy laggards and speed demons – turns out to play a key role in determining the rate of cooling. This degree of deviation from the mean, a property known to statisticians as “kurtosis”, had been overlooked in previous studies, which could explain the poor reproducibility of the Mpemba effect.

Plugging kurtosis into the equations was a game-changer. “We can do analytical calculations to find out how and when the Mpemba effect will occur,” says Lasanta.

When conditions are ideal – if the hot beaker is hot enough than the cold beaker and its molecules are capricious enough to generate high kurtosis – model simulation shows that the hotter sample will cool faster than the colder .

“In fact,” says Lasanta, “we see not only that the hottest can cool faster but also the opposite effect: the colder can heat up faster, what we would call the inverse Mpemba effect.

The next step? Try to verify the simulations – first in the lab, then in the real world. If all goes well, the discovery could have applications in refrigeration and cooling.

Associated reading: Why water levitates on a hot surface

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