Just as hot milk becomes ice cream rather than cold milk
The story of the African Erasto Mpemba is a story of scientific revolution and a struggle to demonstrate the ridiculously unimaginable with experiments. Hot water freezes before water at room temperature or cold (under certain conditions) and although it may seem a thermodynamic absurdity this is as true as a temple.
When Mpemba asked his physics teachers the reason for his observations, more than one told him that he was confused, that it was not possible. Less educated classmates scoffed at the ridiculousness of such a statement, and more educated ones made him out to be misunderstanding Newton’s Cooling Law (and we have to admit that Newton was too smart to dare to defy one of his laws)
<Newton’s Law of Cooling says that the speed at which an object cools is proportional to the difference between its temperature and the temperature of the medium in which the object is located. For those who enter, the equation is T2 = T0 + (T1 – T0) * e (-k * Δt)
where: T2: final temperature, T1: initial temperature, T0: constant ambient temperature, Δt: difference in time between T2 and T1 and k: The constant to be found.>
Other great observers of history such as Aristotle, Descartes and Bacon had noticed the effect. But it was the stubbornness of the young Mpemba who from 1963 to 1969 worked to demonstrate the effect from controlled experiments that the phenomenon was accepted as scientifically possible. However, for the effect to occur, temperature differences of 30°C or more are required, and at high temperatures, e.g. 35°C to 90°C.
Here is an extract from the article presented by Mpemba in 1969:
“My name is Erasto B Mpemba, and I am going to tell you about my discovery, which was due to misusing a refrigerator.
All of you know that it is advisable not to put hot things in a refrigerator, for you somehow shock it; and it will not last long.
In 1963, when I was in form 3 in Magamba Secondary School, Tanzania, I used to make ice-cream. The boys at the school do this by boiling milk, mixing it with sugar and putting it into the freezing chamber in the refrigerator, after it has first cooled nearly to room temperature. A lot of boys make it and there is a rush to get space in the refrigerator. One day after buying milk from the local women, I started boiling it.
Another boy, who had bought some milk for making ice-cream, ran to the refrigerator when he saw me boiling up milk and quickly mixed his milk with sugar and poured it into the ice-tray without boiling it; so that he may not miss his chance. Knowing that if I waited for the boiled milk to cool before placing it in the refrigerator I would lose the last available ice-tray, I decided to risk ruin to the refrigerator on that day by putting hot milk into it.
The other boy and I went back an hour and a half later and found that my tray of milk had frozen into ice-cream while his was still only a thick liquid, not yet frozen. I asked my physics teacher why it happened like that, with the milk that was hot freezing first, and the answer he gave me was that “You were confused, that cannot happen”. Then I believed his answer.
Why does this effect occur?
There are many theories as to why, for example that in a hot container the liquid circulates better, so the water in the central zone moves out faster, cooling down at a higher speed. Another well-known explanation is that the hotter a liquid is, the less dissolved gases it contains (gases make it difficult to freeze). And another common explanation is that warmer water loses more mass than cold water through evaporation.
The truth is that it is not yet known, in 2013 the Royal Society of Chemistry offered a prize of £1,000 to whoever could give the best explanation of why the Mpemba effect worked. They received approximately 22,000 responses, and finally chose the explanation of a chemistry research assistant from the University of Zagreb, Nikola Begovic.
Also in 2013, a major paper by physicists at Nanyang University in Singapore was published, explaining that the effect has to do with how energy is stored in the bonds between hydrogen and water molecules
Remember that each water molecule contains one atom of oxygen and two of hydrogen (H2O), and these atoms are linked by covalents (bonds that atoms have when they share electrons). The hydrogen atoms of one molecule are attracted to the oxygen atoms of the other molecules nearby, through the hydrogen covalent, but at the same time the water molecules repel each other by maintaining a certain distance between them.
Researchers Dr Sun Changqing and Dr Xi Zhanghan have noticed that the distance between the molecules depends on the temperature.
In hot water, the distance is greater and thus also the repulsion, which results in more “tense” hydrogen bonds and therefore stores more energy.
This energy is emitted more quickly in the cooling phase of hot water which has a greater measure than that which has the same amount of water at ambient or cold temperature.
The release of this energy tends, then, to bring the water molecules closer together at a proportional speed that is commensurate with the amount of energy they possess. Thus, the warmer the water, the greater the energy released in the cooling phase, and therefore the greater the speed of freezing.
Nikola Bregovic, the winner of the Royal Society of Chemistry contest, conducted several experiments and suggests that the convection effect was probably responsible for the Mpemba effect.
There is still a lot of discussion and controversy about this effect, and although I find everything tremendously interesting, I can’t help but think how almost paradoxical it is that sometimes empirical knowledge must struggle with “scientific” knowledge. The same year of his discovery in 1963 Mpemba visited a friend who lived in another city and who worked selling ice cream and observed that his friend put the hot mixes in the refrigerator to prepare the ice cream, when he asked him why his friend answered “My brother has been making ice cream for five years and he says that this way they are made faster”.