This seemingly simple question is still the source of disagreement. It sounds like an easy enough experiment. Take two cups of water (one hot and one cold). Place both in a freezer and see which one freezes first. How difficult can it be? Common sense would suggest that the colder tem­pera­ture water will freeze first. Not so fast. While a logical conclusion, it turns out that hot water can freeze before cooler water under certain conditions.

This apparent quirk of nature is the ‘Mpemba effect’, named after the Tanzanian high school student, Erasto Mpemba, who first observed it in 1963.

Evaporation is the strong­est candidate to explain the Mpemba effect. As hot wa­ter placed in an open container begins to cool, the over­all mass decreases as some water evaporates. With less water to freeze, the process can take less time. But this does not always work, especially when using closed containers that prevent evaporated water from escaping.

In warmer water there is less dissolved gas which can reduce its ability to conduct heat, allowing it to cool faster.

Others attribute this phenomenon to the different temperatures at the top and bottom of the container (for the hot water), resulting in convection currents which accelerate the cooling process.

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