Hot water freezes faster than cold water

Hot water freezes faster than cold water

When we think about it, it seems self-evident: in equal volumes, hot water freezes more slowly than cold water. Because to reach 0 ° C, the water already cold to lose less heat, less energy to dissipate, as hot water. This makes sense somehow.

Yet thermodynamics provide something completely different. This is the Mpemba effect, named after its discoverer Tanzania in 1963.

The effect explains that hot water freezes faster than cold water, and for several reasons. First, because of a difference of volume. Some chemists argue that the evaporation of water in the hot sample is responsible for a loss of volume. As the hot water
has less volume, it freezes faster than cold water. But it is highly questionable because the loss of volume is not quite consistent.

Then, cold water has the ability to enter a particular state, super cooling: it adopts a met stable behaviour (between liquid and solid) that allows it to remain liquid to very low temperatures (-40 ° C in the atmosphere; -5 ° C in the freezer).To make this
possible, it is necessary that water be "calm". However, hot water, in fact, is not quiet: it is more than cold water, subjected to convection due to temperature differences within the liquid. Therefore difficult for the water to get in a met stable equilibrium:
it freezes at 0 ° C.

In addition, the gas dissolved in water (mainly CO2 and O2) lower the freezing cold water. In hot water, it’s different because these gases escape because heat from the hot water makes them insoluble. This is why cold water suffusions much more than hot water.

That’s not all. The hot water molecules have enough energy to leave the body fluid as a gas vapor (thus, she withdrew from the heat energy). Cold water molecules have little energy, at least not enough to leave the liquid. And hot water loses heat faster. This
means that it loses its energy faster than cold water and reaches its freezing point first.

This makes it difficult or impossible to consider one of the phenomena independently of the others, and make it alone, responsible for the Mpemba effect. Nevertheless, the existence of these different causes makes it plausible that, under certain circumstances,
hot water can freeze faster than cold water. This beam of explanation should convince sceptics to accept the sum of experimental observations.

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Posted by Fouad Ashraf on Dec 1 2011. Filed under Sci-Tech. You can follow any responses to this entry through the RSS 2.0. You can leave a response or trackback to this entry