From Keith Ross
Marcus Chown reports on James Brownridge’s theory that the reason hot water freezes faster than cold – the Mpemba effect – is a consequence of supercooling in the water that started off cold (27 March, p 10). However, after many years of getting my student teachers to investigate the phenomenon, the neatest explanation I have found centres on the role of convection.
We often compared two samples, one of which started hotter than the other, and found that the hot water contained more ice than the cold. Crucially, they were both partly frozen. This discounts the supercooling explanation because once the first ice crystals had appeared in the supercooled water it would have quickly formed more ice than had formed in the hot water.
A better explanation relates to the rates at which heat can be transferred away from the water. We postulated that when placed in a freezer, cold water would quickly separate into three layers: water at 4 °C at the bottom of the flask, 0 °C ice at the top and room temperature water in between. This separation would leave conduction as the only method available for further heat loss.
In the hot water system, however, convection currents would persist, preventing this static phase from establishing and allowing a much faster heat transfer to the freezer.
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It would be an interesting check on hypotheses about the Mpemba effect if the test were to be carried out under zero gravity, which would result in suppression of thermal convection.
Totnes, Devon, UK
