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Can Hot Water Freeze Faster Than Cold Water?

Sometimes. Under some conditions, initially hot water can freeze faster than initially cold water โ€” a phenomenon called the Mpemba effect. It does not always happen: the outcome depends on the container, dissolved gases, evaporation, convection, and supercooling, and scientists still debate which mechanism matters most.

Quick facts

Why the setup decides the winner

Start with two seemingly identical samples and the race can flip just by changing details. Hot water evaporates more, so there may simply be less of it left to freeze. Heating also drives dissolved gases out of water, and gases in the water change how it cools and freezes. A hot sample sets up stronger convection currents that keep heat moving to the surface. And cold water can supercool โ€” drop below its freezing point without turning to ice โ€” which delays it in ways that have little to do with its starting temperature.

Because so many variables can each tip the result, experiments are hard to replicate cleanly, and different careful studies have pointed at different leading causes. That is why the honest summary is not "hot beats cold," but "hot can beat cold, under the right conditions, for reasons still being argued about."

How to think about trying it at home

A kitchen freezer test can be fun, but it is easy to fool yourself: trays in different positions, different water levels, a freezer cycling its compressor, or one sample supercooling can all decide the result before physics gets a say. If you try it, keep everything identical except starting temperature โ€” and be ready for the boring outcome, because cold water winning is also a perfectly normal result.

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Fascinating facts you didn't know. Come learn with me โ€” no pressure.

Sources: Royal Society of Chemistry (Mpemba effect competition and papers); Scientific American (coverage of the Mpemba effect and the debate over its mechanism).