In 1963, a Tanzanian schoolboy named Erasto Mpemba noticed something strange: while making ice cream, he took a boiled milk-and-sugar mixture and froze it before it had cooled down. After about 90 minutes, his mixture had frozen into ice cream but his classmates’ mixture was still liquid. Mpemba’s discovery became highly publicised but there’s still a debate over why it happened:
Does hot water freeze faster than cold water? […] What makes this question so hard to nail down? Among many of the issues complicating exactly how to measure such a thing is that water frankly has some odd properties; it is less dense as a solid, and it is also possible for its solid and liquid phases to exist at the same temperature. Also, water in the process of freezing is not in equilibrium, and how exactly things act as they relax into equilibrium is a process for which — physics-wise — we lack a good theory. Practically speaking, it’s also a challenge how to even accurately and meaningfully measure the temperature of a system that is not in equilibrium.
But there is experimental evidence showing that the Mpemba effect can occur, at least in principle. How this can happen seems to come down to the idea that a hot system (having more energy) is able to occupy and explore more configurations, potentially triggering states that act as a kind of shortcut or bypass to a final equilibrium. In this way, something that starts further away from final equilibrium could overtake something starting from closer.
But does the Mpemba effect actually exist — for example, in water — in a meaningful way? Not everyone is convinced, but if nothing else, it has sure driven a lot of research into nonequilibrium systems.
via Hackaday
The top Google search result (not indicative of anything but important because it gets a featured snippet which at least makes it visually prominent) is a paper saying hot water doesn’t freeze faster than cold water:
The Mpemba effect is the name given to the assertion that it is quicker to cool water to a given temperature when the initial temperature is higher. This assertion seems counter-intuitive and yet references to the effect go back at least to the writings of Aristotle. Indeed, at first thought one might consider the effect to breach fundamental thermodynamic laws, but we show that this is not the case. We go on to examine the available evidence for the Mpemba effect and carry out our own experiments by cooling water in carefully controlled conditions. We conclude, somewhat sadly, that there is no evidence to support meaningful observations of the Mpemba effect.
Abstract from ‘Questioning the Mpemba effect: hot water does not cool more quickly than cold‘
But there are plenty of over ideas that say it can, under certain conditions such as ‘if you have two non-magnetic systems above the Curie temperature’ or if ‘the components of a hotter system, by virtue of having more energy, are able to explore more possible configurations and therefore discover states that act as a sort of bypass, allowing the hot system to overtake a cool one as both dropped toward a colder final state’.
It’s not so much the proof or disproof that matters to me but the fact that a young Black African teenager started decades of research and, as Quartz put it, ‘leading physicists toward new theories about how substances relax to equilibrium’.