In 1963, a Tanzanian student named Erasto Mpemba noticed something odd while making ice cream. When he froze his boiling-hot mixture, it cooled faster than his classmate’s cooler mixture. This phenomenon was later published in a paper by Mpemba in 1969, and became known as the Mpemba effect. But to this day, no one is quite sure how it works.
Now, scientists from the Southern Methodist University in Texas and Nanjing University in China have published a new paper in the Journal of Chemical Theory and Computation that seeks to provide an answer. They suggest that the links between hydrogen atoms and oxygen atoms of neighboring water molecules might be the cause.
“We see that hydrogen bonds change when warming up water,” Dieter Cremer from the Southern Methodist University, one of the researchers, told ScienceNews.
He added that in higher temperatures, more hydrogen bonds were strong because the weaker ones were broken down. This caused groups of molecules to form into fragments that could realign into the crystalline structure of ice. For colder water, the bonds must first be broken before this can take place.
However, the idea that hot water can freeze faster than cold water remains controversial. Another recent article in Scientific Reports from November 2016 said there was “no evidence to support meaningful observations of the Mpemba effect.” The authors added they were “not gladdened by such a conclusion, indeed quite the opposite,” as the effect had proven to be engaging to “people of all ages and backgrounds.”
One major problem is that the effect is difficult to reproduce, although that hasn’t stopped plenty of theories being put forward for how it might work. One is that the hot water may evaporate, which reduces the mass and amount of water to be frozen. Another is that water at lower temperatures freeze from the top, while warm water freezes from the bottom, although this is contentious.
It’s certainly still an interesting topic. Forbes notes that in 2012, the Royal Society of Chemistry even held a competition to find the best explanation for the effect – with the winner thinking that “supercooling” was at play.
But exactly why it occurs, or even if the effect is real at all, isn’t clear. Perhaps this latest paper will offer a new solution, but the case is far from settled for now.