Named after Erasto Mpemba, is the assertion that, in some circumstances, warmer water can freeze faster than colder water. Although there is anecdotal support for the effect, there is no agreement on exactly what the effect is and under what circumstances it occurs. There have been reports of similar phenomena since ancient times, although with insufficient detail for the claims to be replicated.
The phenomenon seems contrary to intuition, but a number of possible explanations for the effect have been proposed. Further investigations will need to decide on a precise definition of “freezing” and control a vast number of starting parameters in order to confirm or explain the effect.
A reviewer for Physics World writes, “Even if the Mpemba effect is real — if hot water can sometimes freeze more quickly than cold — it is not clear whether the explanation would be trivial or illuminating.” He pointed out that investigations of the phenomenon need to control a large number of initial parameters (including type and initial temperature of the water, dissolved gas and other impurities, and size, shape and material of the container, and temperature of the refrigerator) and need to settle on a particular method of establishing the time of freezing, all of which might affect the presence or absence of the Mpemba effect. The required vast multidimensional array of experiments might explain why the effect is not yet understood.
New Scientist recommends starting the experiment with containers at 35 °C (95 °F) and 5 °C (41 °F) to maximize the effect. In a related study, it was found that freezer temperature also affects the probability of observing the Mpemba phenomena as well container temperature. For a liquid bath freezer, a temperature range of −3 °C (27 °F) to −8 °C (18 °F) was recommended.
In 2012, the Royal Society of Chemistry held a competition calling for papers offering explanations to the Mpemba effect. More than 22,000 people entered and Erasto Mpemba himself announced Nikola Bregović’s paper as the winner.