Hot water freezes faster than cold: True/False?

Physicists used to laugh at this idea. I imagine that words like balderdash and piffle were employed: this was back in the early twentieth century when physicists could really sling an insult with panache. It wasn’t until the sixties that a Tanzanian high school student named Erasto Mpemba forced them to acknowledge the truth and the phenomenon still bears his name.

The argument against the Mpemba effect is pretty simple. Imagine two containers of water in a freezer, one hot the other cold. Before the hot water reaches freezing point it must pass through all the temperatures in between, including the starting temperature of the cold water. So if the cold water takes, say, half an hour to freeze, the hot water must take that half an hour plus however long it takes to cool down to where the cold water started. The cold water has an insurmountable head start.

But there is a disguised assumption in there. Who says that cooled hot water is exactly the same as cold water that was never heated? One difference we can see is that by the time the hot water cools down there is less of it. That’s because hot water evaporates readily — this is one of the ways it cools down — and the less water there is the faster it will freeze.

There’s more to it than that, though, as the effect has been reported with closed containers that prevent evaporation. There are at least half a dozen factors that impinge upon the Mpemba effect and this has hindered efforts to discover a thorough explanation. The study of ice and water may sound like scientific make-work but in reality we don’t yet have a full grasp of their nature and behaviour.

The Mpemba effect is real, though, even if we can’t explain it right now. Science progresses this way, with new information disproving the current theory. Some scientists are reluctant to abandon familiar ideas — they are only human after all — but they come round in the end. Otherwise they find themselves left out in the cold.

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