Probably you’re wondering if the words in the title make sense. But before we jump into the definition of the Mpemba Effect, let’s hear (read) the story of a Tanzanian teenager in the 1960s, Erasto Mpemba.
During his cookery lessons in school, our friend Mpemba had observed something peculiar while making ice-cream – The hot ice-cream mixture from the stove froze faster than the cold mix. Confused and driven by curiosity, he decided to try the same with water and observed something strange yet again. Yes, as per his observations, hot water froze faster than the cold one. Luckily, physicist Dr. Denis Osborne visited his school for a lecture and Mpemba posed this question to him even after being ridiculed by his classmates. Later in 1969, Osborne co-authored a paper with Mpemba describing the Mpemba effect! It became a quite confusing but accepted phenomenon at that time.
Now, to put it very crudely, Mpemba effect is the peculiar behaviour that water shows wherein hot water freezes faster than cold water. One interesting fact is that this wasn’t the first time someone had made this observation. There are three historical references always cited when describing this effect. Scientists like Aristotle, Francis Bacon and René Descartes have made similar observations in the past. Quoting Aristotle, “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.”
So, we understand what the effect is and its historical perspective, but we still don’t know why this entire anomaly is taking place. A very common sentence I have often come across while reading on topics like these is “It is a nice story. But is it real?” Well, in this case, we don’t know! Scientists have conflicting views not only on the reasons of this process taking place but also on whether the thing is if at all real!
There have been quite a number of explanations put forward by scientists in order to explain this phenomenon. According to an article in JCTC, the answer lies in the nature of Hydrogen bonds in the water where the ones in warm water are weaker. There are other explanations like crystallization, the presence of solutes, etc. Interestingly in 2012, the Royal Society of Chemistry held a contest calling for explanations to the Mpemba effect. There were more than 22k participants and Nikola Bregović was selected as the winner by Erasto Mpemba himself. He suggested four possible reasons which impacted the cooling of water – Supercooling, Enhanced Convection, Dissolved Gases and Evaporation of water. “A colder sample gets supercooled rather than frozen, and enhanced convection in the warmer sample speeds up cooling by maintaining the heat gradient on the container walls”.
So, where does the problem lie? Firstly, it’s the definition. According to many scientists, the definition of the effect is not well established and has loopholes. Many claim the lack of reproducibility of data as a concern. As per the words of Henry Burridge, “In 50 years no single person has ever managed to repeat anything like that data.” He was referring to what Mpemba and Osborne had published in 1969. Henry Burridge has also co-authored a very controversial paper in 2016 which completely writes off the Mpemba effect! Quoting his words, “Water is a peculiar fluid, but we don’t believe the Mpemba effect exists in a meaningful way. It would have been far more exciting to conclude that it was true.”
The scientific community, however, is not ready to erase the Mpemba Effect even though they have conflicting opinions. I hope that through this article, I was able to convey to you what the Mpemba Effect is! I believe that scientists will keep seeking explanations and we’ll keep getting amazed at what a small molecule like Water can do!
Images: Google, Pexels
By Raibat Sarker, Department of Chemistry, IISER Bhopal.
About the author: Raibat is a 3rd-year Integrated MS student at the Department of Chemistry, IISER Bhopal and a KVPY fellow. Apart from being a big foodie and FCB fan, he loves trekking and camping. When he’s not sciencing, eating, or complaining about his bad luck, you will find him hooked into novels or planning his next trek! Also, he is associated with The Qrius Rhino.