If two glasses of hot and cold water are left in the refrigerator, Common sense suggests that the colder water will freeze faster. However, luminaries including Aristotle, Rene Descartes, and Sir Francis Bacon have noticed that hot water may cool more quickly.
The modern phrase for hot water freezing faster than cold water is the Mpemba effect, named after Erasto Mpemba, a Tanzanian teenager who, along with the physicist Denis Osborne, performed the first systematic scientific studies regarding this in the 1960s. While they successfully observe the effect, follow-up experiments have consistently failed to replicate that outcome. Precision experiments to analyze freezing can be influenced by several subtle details, and researchers always have trouble understanding if they have accounted for all confounding variables.
Since the last few years, as the controversy continues about whether the Mpemba effect takes place in water, the phenomenon has been detected in other substances as well. These new directions assist researchers in looking into the complicated dynamics of systems that are out of thermodynamic stability. A group of physicists modeling out-of-equilibrium systems has anticipated that the Mpemba effect should occur in various materials. Recent experiments have been conducted to verify these ideas.
Yet the most common substance of all, i.e., water, is proving to be the hardest.
“A glass of water stuck in a freezer seems simple,” said John Bechhoefer, a physicist at Simon Fraser University in Canada whose recent experiments are the most vital observations of the Mpemba effect. “But it’s actually not so simple once you start thinking about it.”