Chemistry World Blog

Over the last few months, the RSC has been running a competition, asking people to try and explain the Mpemba effect – why does hot water freeze faster than cold water?

The effect has puzzled scientists throughout history, but was most recently brought to light again in the 1960s by Eristo Mpemba, a student from Tanzania who challenged the received wisdom of his teachers and ended up writing a paper on the phenomenon with a local university professor, Denis Osborne.

Over 22,000 people from 122 countries submitted their theories and potential explanations for the effect, ranging from the hare-brained and humorous to more thorough and considered arguments. These entries were then whittled down to 11 by an international panel of judges and a public vote.

On Friday of last week, the RSC held an awards ceremony to announce the winner. Mpemba himself flew in from Tanzania to attend, and Osborne also joined the gathering at Burlington House in London.

And the winner? Nikola Bregovic, a PhD student in physical chemistry at the University of Zagreb, Croatia. Not satisfied with theory and conjecture, Bregovic took to both the library and the lab, performing his own experiments and research to try and work out which of the many factors proposed to cause the Mpemba effect were significant.

You can read his submission in full in this paper, but it appears that the biggest factor affecting whether hot water will freeze faster than cold water is the degree to which the water can be supercooled – cooled below its nominal freezing point without actually freezing. Under certain conditions, it appears that heating the water can raise the nucleation temperature of the water, meaning that it effectively freezes at a higher temperature than the cold water. Convection – which will be more pronounced in the hot water – also plays a part, and the underlying fundamental problems underline the complexity of water’s chemistry. As Bregovic says in his conclusion, ‘Once again this small, simple molecule amazes and intrigues us with its magic.’

And how will he spend the £1000 prize money? A night out with his supporters and labmates to celebrate, and a trip to the mountains in the spring with the leftovers.