The Mpemba effect

That hot water freezes quicker than cool water had been noted by many observers including Aristotle, Bacon and Descartes. However, the effect has come to be named after the schoolboy who brought the phenomenon back to the attention of scientists. In the 1960s, Erasto Mpemba, then a secondary school student in Tanzania, was making ice cream and noted that boiled milk placed in the freezer froze faster than milk at room temperature. He approached his science teacher who said he must have been confused and mocked his answers in class, describing his ideas as ‘Mpemba physics’. When a professor from a local university visited his school, Mpemba raised the issue. The scientist asked a technician to confirm the result and the academic and Mpemba published a co-authored paper on the effect.

This counter-intuitive result has led to the publication of several papers on the phenomenon. There is an on-going debate about the causes of the Mpemba effect. Some researchers argue that the properties of hydrogen bonds explain the phenomenon, whilst others claim that the effect is merely an artefact of experimental technique.