Science
Why does hot water sometimes freeze faster than cold?
- It is disputed and still not settled
- It always does
- It never does
The answer is It is disputed and still not settled. The Mpemba effect - reported for centuries, still argued over.
It sometimes does, and nobody has settled why - which is the honest answer to a question that sounds like it should have one. It is called the Mpemba effect, after a Tanzanian schoolboy who noticed in 1963 that his hot ice cream mix froze before his classmates' cool mix and was told by his teacher that he was mistaken.
He was not the first. Aristotle described it, and so did Bacon and Descartes. Erasto Mpemba's contribution was to keep insisting, and to publish with a physicist who took him seriously enough to test it.
Proposed explanations include evaporation reducing the volume, dissolved gases driven out by heating, convection currents, frost under the container melting, and the way water's hydrogen bonds store energy. Each explains some experiments and not others, and a careful 2016 study failed to observe the effect at all under controlled conditions, arguing it depends on where in the container you measure. A genuinely open question about freezing water is a good advertisement for how much ordinary physics is unfinished.