This question might seem ridiculous since normal human sense would make this assumption look impossible. Nevertheless this extremely counterintuitive phenomenon is real and does occur under certain conditions. The paradox observation was described several times in human history and was first mentioned by Aristotle in 300 B.C. followed by physicists in the medieval era and finally by Erasto B. Mpemba, a Tanzanian scientist. He found this strange behavior at the age of 13 while making ice-cream at school. Mpemba put hot milk mixed with sugar in the freezer at the same time when one of his classmates put in the same mixture, which was at room temperature. When they looked inside the freezer a little while later, Mpemba’s ice-cream was frozen and the other kid’s one was not. Since that time, this phenomenon was called “Mpemba effect”.
But up until now no final solution for this problem has been found. Of course temperature alone cannot be the only factor in these experiments, but other ones like shape and size of the container, shape and size of the refrigerator unit, the gas and other impurity content in the water, the initial difference in temperature of the two samples, to name just a few, might be part of the explanation. This immense variation in experimental setup makes it hard to obtain reproducible results. Nonetheless several mechanisms for this behavior like evaporation and convection are discussed. They are conclusive, but not a single of these mechanisms can explain the Mpemba effect completely and for all circumstances. It seems as if we had to look for a complete picture and not just for “the” mechanism.
The Mpemba effect is a great example that odd-looking results in science are not always a failure. So share your problems with the community so that they can be solved. That is what JUnQ is for, because science never fails!
Andreas Neidlinger
Read more:
http://iopscience.iop.org/0031-9120/4/3/312/
http://www.desy.de/user/projects/Physics/General/hot_water.html