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Did you know that, under some conditions, hot water can freeze faster than cooler water? It doesn't appear to make any sense, I know. On its way to freezing, surely hot water has to pass through a point where its temperature is the same as the cooler water and after that point both should freeze at the same rate? And that would mean that the cold would freeze quicker than the hot? Right?

Despite the fact that this might not seem right at first, numerous experiments have shown that it is the case. It has, in fact, been well documented in history by Aristotle, Bacon, and Descartes but it wasn't till it was rediscovered by a Tanzanian school boy called Mpemba that scientists really got involved in the debate. This effect still leaves scientists pretty divided and no one really knows why it happens.

What happened was that, one day, Mpemba was making ice-cream and he noticed that hotter liquids could freeze faster than cold. So he told his teachers and they, most probably being far too concerned with wanting to eat the ice-cream in question, chose to not believe him. It wasn't till later when visiting Britain’s repeated his experiments that people realized he hadn't been overdosing on sugar and was actually right.

So, if you start with two identical volumes of water and freeze them in exactly the same way with the only difference being that one starts at a higher temperature, how on Earth can the hot one freeze sooner than the cold? There are a number of theories that can possibly explain this phenomenon, although it may be a combination of effects each playing a role.

Firstly, we have evaporation. Some of the hot water could be lost through evaporation meaning that there is less to freeze and it can therefore freeze faster than the larger volume of cooler water. This however doesn't explain what happens in closed containers so can only be part of the solution. There is also the fact that warmer water is less dense and there are therefore less molecules of warm water as there are in an equal volume of cooler water. Less molecules = less total heat loss required to freeze the water. But again, this effect on its own doesn't seem great enough to fully explain the Mpemba effect.

Next we have the possibility that the heated water is somehow changed in its properties. Hot water can hold less dissolved gas than cold (think how much gas bubbles out of water just before it boils) and the presence of this gas could be lowering the effective freezing temperature of the cold water. Or the loss of the gas in the warmer water could allow convection currents to be set up more easily.

Convection currents in water turns out to be a nice explanation for the Mpemba effect. The idea is that, in hot water, currents are set up with the hot water travelling up the middle of the ice-cream and cold down the sides so the top is warmer than the bottom. This means that even when the hot water cools to the same average temperature as the cold, the temperature distribution of the water may not be equal. In other words, 30ºC starting temperature water will uniformly be at this temperature while cooled water that started much hotter can have an average 30ºC temperature but could include some water that is hotter and some that is colder. The convection currents set up in the warmer water could in theory stay in place until the water freezes, meaning it loses heat much faster than the water that started at a cooler temperature.

Supercooling could also be involved. Supercooling can cause a liquid to remain a liquid past its normal freezing temperature. If the cooler water supercools while the warmer doesn't, it could take longer to become a solid, explaining the Mpemba effect.

Despite several experiments showing the Mpemba effect does exist, it will only work under certain conditions. All my attempts so far have been unsuccessful but this doesn't mean the effect doesn't exist. Different temperatures, starting temperatures and volumes of water will make a difference to the experiment. Why not try it yourself?

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