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Today I learned hot water sometimes freezes faster than cold water. Although there isn't one, conclusive scientific explanation for the phenomenon, one of the theories for why this occurs is that hot water loses mass to evaporation. When something has less mass, less energy is required for it to freeze.

It is known as the Mpemba effect because a Tanzanian secondary school student Erasto Mpemba noticed the phenomenon while using boiled milk as part of a mixture to make ice cream. He then reintroduced it to the modern scientific community. However, the effect was described long ago by people like Aristotle and Descartes, among others.

atomic structure of a water (or dihydrogen monoxide)As an aside, Mpemba was mocked by his teacher for claiming hot water freezes faster than cold water because it seemed to contradict thermodynamics. Moral of the story? Don't be so fast to dismiss the observations of a non-scientist just because they don't agree with what you think you know.

In fact, that lesson probably applies to most academic disciplines and, perhaps, all of human knowledge. Skepticism of people's claims is important.

The Mpemba effect occurs when two bodies of water, identical in every way, except that one is at a higher temperature than the other, are exposed to identical subzero surroundings, and the initially hotter water freezes first. The effect appears theoretically impossible at first sight. Nevertheless, it has been observed in numerous experiments, and we will see that it is in fact possible.

Readers who are quite certain that the effect is forbidden by the laws of thermodynamics should pause for a moment to do two things. First, to try to explain, as precisely as possible, why it is impossible. And second, to decide how to respond to a non-scientist who insists that they have observed the Mpemba effect in a home experiment. Whether or not the effect is real, careful consideration of these tasks will bring up a wealth of important issues about the scientific method, that can be understood and discussed by students without any knowledge of advanced physics...

To see how the effect might occur, it is useful to carefully think about why the effect appears impossible. The careful reader may already have come up with a proof of impossibility that goes something like this:

Suppose that the initial temperatures for the hot and cold water are 70°C and 30°C. Then the 70°C must first cool to 30°C, after which it must do everything the 30°C water must do. Since the 70°C water has to do everything the 30°C water must do, plus a little more, it must take longer to freeze.

A good way of systematically analyzing the Mpemba effect is to think about why this proof is wrong.

The problem with this proof is that it implicitly assumes that the water is completely characterized by a single parameter - the temperature. We need to think of a parameter that might change during the course of the experiment; then, the 70°C water cooled to 30°C will not be the same as the water initially at 30°C.

One possible parameter is the mass of the water. Both bodies of water initially have the same mass. But if the initially hotter water loses mass to evaporation, then the 70°C water cooled to 30°C will, having less mass, be easier to freeze (i.e. less energy will need to be removed to cool and freeze it). This is one of the strongest theoretical explanations for the Mpemba effect. Kell numerically integrated the heat loss equations, assuming that the cooling was by evaporation alone, and that the mass lost to evaporation never recondensed - he found that with these assumptions, there were initial temperatures where the hot water would freeze faster than the cold water. But this does not prove that evaporative cooling is the only factor behind the Mpemba effect. A number of experimenters have claimed that the amount of mass they lost to evaporation was insufficient to explain their results. And Wojciechowski et. al. observed the Mpemba efect in a closed container, which further suggests that evaporative cooling is not the sole cause of the effect.

Hot water can freeze faster than cold?!?

What did you learn recently?


Original posting by Braincrave Second Life staff on Jun 14, 2011 at http://www.braincrave.com/viewblog.php?id=577

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