The Mpemba Effect
What would you say if I told you that sometimes, warm water freezes faster than cold water? Well, it happens, and this strange phenomena is called the Mpemba Effect.
A little history
For centuries, scientists and philosophers have noted that under certain hard-to-define conditions, warm water will freeze faster than cold water when placed under the exact same conditions.
“The fact that the water has previously been warmed contributes to its freezing quickly: for so it cools sooner. Hence many people, when they want to cool water quickly, begin by putting it in the sun. So the inhabitants of Pontus when they encamp on the ice to fish (they cut a hole in the ice and then fish) pour warm water round their reeds that it may freeze the quicker, for they use the ice like lead to fix the reeds.”[5] — Aristotle
“slightly tepid water freezes more easily than that which is utterly cold.”[6] — Francis Bacon
“One can see by experience that water that has been kept on a fire for a long time freezes faster than other, the reason being that those of its particles that are least able to stop bending evaporate while the water is being heated.”[7] — Rene Descartes
The first person who documented this in the modern world in a more systematic way was Erasto Mpemba. When he was just a high schooler, he observed that ice cream mix which was hot in his cooking class would freeze quicker than ice cream which started out cold.
Intrigued, Mpemba investigated further. When a physics professor from the local university named Dr. Denis G. Osborne came to visit the high school one day, Mpemba asked the professor: “If you take two similar containers with equal volumes of water, one at 35 °C (95 °F) and the other at 100 °C (212 °F), and put them into a freezer, the one that started at 100 °C (212 °F) freezes first. Why?”
At first, Mpemba was ridiculed by his class for asking such a seemingly stupid question. But Osborne later followed up on Mpemba’s question and confirmed Mpemba’s findings. From there, he worked with Mpemba to publish a paper on the effect. And from that paper the effect was named.
The specific experiment they did was placing 70 ml samples of water in 100 ml beakers in the ice box of a refrigerator. They repeated the experiment for many different initial temperatures, and found that the longest freezing time occurred with an initial temperature of around 77 Fahrenheit, while the shortest freezing time was around 194 F. They specifically ruled out the loss of liquid volume by evaporation as a potential factor.
Potential Causes
So, what causes this? A bunch of potential explanations have been proposed — below are some of the ones that Wikipedia mentions:
- Convection: Accelerating heat transfers. Reduction of water density below 4 °C (39 °F) tends to suppress the convection currents that cool the lower part of the liquid mass; the lower density of hot water would reduce this effect, perhaps sustaining the more rapid initial cooling. Higher convection in the warmer water may also spread ice crystals around faster.[13]
- Frost: Has insulating effects. The lower temperature water will tend to freeze from the top, reducing further heat loss by radiation and air convection, while the warmer water will tend to freeze from the bottom and sides because of water convection. This is disputed as there are experiments that account for this factor.[4]
- Thermal conductivity: The container of hotter liquid may melt through a layer of frost that is acting as an insulator under the container (frost is an insulator, as mentioned above), allowing the container to come into direct contact with a much colder lower layer that the frost formed on (ice, refrigeration coils, etc.) The container now rests on a much colder surface (or one better at removing heat, such as refrigeration coils) than the originally colder water, and so cools far faster from this point on.
- Dissolved gases: Cold water can contain more dissolved gases than hot water, which may somehow change the properties of the water with respect to convection currents, a proposition that has some experimental support but no theoretical explanation.[4]
- Crystallization: Another explanation suggests that the relatively higher population of water hexamer states in warm water might be responsible for the faster crystallization.[15]
- Entropy: Water after being heated and recooled at the initial temperature,has more entropy than before it was heated[16]
As you can likely tell from the huge number of possible explanations, the jury is still out on what exactly causes the effect. This is compounded by the fact that there are some arguments about how exactly to define freezing (is it when it starts freezing or finishes freezing), as well as by the fact that there are a lot of potential parameters (the temperature of the cold environment impacts the effect, for instance).
That said, in 2012, the Royal Society of Chemistry hosted a competition to see who could offer the best explanation. Over 22,000 people entered, and Mpemba himself graded the results. He eventually chose Nikola Bregović as the winner, who suggested that the effect primarily resulted from supercooling (colder water gets supercooled rather than frozen) and better convection currents in the hot water allowing a more rapid transfer of temperature.
One final possible explanation came recently, in 2016. Some scientists who used “vibrational spectroscopy” found that warming water causes the distribution of hydrogen bonds to change in the water, creating a higher % of strong hydrogen bonds. The existence of many strong hydrogen bond clusters evidently facilitates the formation of ice.
