Why does hot water freeze faster

His question to guest lecturer Dr. Denis G. It's completely counterintuitive and seems to violate the basic laws of thermodynamics. To be clear, what we're saying here is that under certain conditions, the total time it takes for a volume of warm water to freeze will be smaller than the total time it takes for an equal volume of cold water to freeze, given the exact same external temperature.

It's a really strange thing. I mean, at some point in the process, doesn't the warm water reach the exact same initial state as the cold water? And if so, why does that cold-water-that-was-recently-hot freeze faster than the water that started out cold?

It's left folks scratching their heads or outright denying its existence for decades. Since then, numerous explanations have been put forth to try and explain the phenomenon, but none have been much more than plausible-sounding theories.

Here are a few of them:. Theory: Convection currents in the warm water caused by large temperature differentials will cause it to cool more rapidly, and those convection currents continue even after the water has dropped to the same temperature as the cooler water, thus allowing it to overtake the cooler water in freezing. Theory: Hot water evaporates.

Less water left behind means less water to freeze. Theory: Hot water creates convection patterns in the air inside a freezer, which increases its cooling efficiency. Theory: Cold water freezes in a layer on top, creating insulation and preventing the rest from cooling very fast.

The experimental problems are large because there are so many variable to control—aside form starting temperatures, there's also the shape of the freezer, the volume and shape of the container, the insulative properties of the container, the dissolved solids in the water, etc.

The new paper claims that there's actually a chemical explanation for the effect, and one that mathematically fits observed data—as far as I know, the first explanation to be able to do so. Water molecules consist of two hydrogen molecules attached to an oxygen molecule primarily through strong covalent bonds.

And evaporation may not be the only reason the water can freeze more quickly. There may be less dissolved gas in the warmer water, which can reduce its ability to conduct heat, allowing it to cool faster. However, Polish physicists in the s were unable to conclusively demonstrate this relationship. A non-uniform temperature distribution in the water may also explain the Mpemba effect. Hot water rises to the top of a container before it escapes, displacing the cold water beneath it and creating a "hot top.

These currents are a popular form of heat transfer in liquids and gases, occurring in the ocean and also in radiators that warm a chilly room. With the cooler water at the bottom, this uneven temperature distribution creates convection currents that accelerate the cooling process. Are there materials that can absorb heat without becoming hot? How can a snail crawl upside-down on the underside of the surface of a pond? Why does structural behavior change in different types of soil?

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