Does hot water freeze faster than cold?

I once read that hot water freezes faster than cold water. Understandably I threw my arms up in the air and started ranting about the laws of thermodynamics; safe in the knowledge that whoever wrote such a thing was an A-grade idiot.

Except that it’s true … kind of.

It turns out there’s something called the Mpemba effect, which means that sometimes hot water will freeze faster than cold. That doesn’t mean a cup of boiling water straight from your kettle will freeze quicker than a cup of fresh glacial melt water when thrown into a deep freeze, but if the temperatures of the two samples, their volumes and the freezing temperature used fall within an optimal range, it will work – and your kitchen probably has everything you need to create the right conditions.

I tried it myself by filling two ice cube trays and putting them side by side in our freezer – one was filled straight from the hot tap and the other from the cold tap. I checked on them every hour or so, and sure enough, after a while the hot side had frozen solid while the cold side had only a thin surface layer of ice that was easy to poke through.

So what causes it? Well, if you look at this Wikipedia article about the Mpemba effect you can see there’s no real consensus. Evaporation, convection currents and the insulating properties of frost are all likely culprits but none of these appear to account for the entire effect. It’s likely a combination of many things.

I’d love to hear from you if you try it out for yourself. And if all this isn’t weird enough for you, check out the Leidenfrost effect which allows water to sometimes vapourise faster on a colder surface than a hotter surface.

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