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Name: Hannah
Status: other
Grade: other
Location: N/A
Country: N/A
Date: February 2007

Question:
Is it possible for water to reach -20 Celsius or colder without freezing? Is there a minimum temperature that water supercools and if yes, what would that be?



Replies:
This is not such an easy question to answer definitively, because water below 0 C. is unstable, so anything that provides a growth site starts supercooled water to turn to ice. This could be a dust particle, or even a random fluctuation in the density of the water itself. The lowest temperature that has been observed is about -40 C. However, this is not an absolute lower limit "in principle". What happens is this. As the temperature decreases, the viscosity of water increases dramatically --it becomes more like syrup or honey so the water molecules cannot move around quickly enough to form crystals. This behavior also depends upon the pressure applied to the water. Under the right conditions (and luck) water can be cooled to form a glassy state, which is unstable energetically, but cannot release the energy because the water molecules are "frozen" in place.

You can demonstrate this effect using glycerin instead of water. The melting point of glycerin is 14 C., but almost no one has seen crystalline glycerin because its viscosity increases so much as the temperature is reduced below its melting point. You may also have seen another example without realizing it. If you leave a jar of honey on the shelf for a long time you frequently see that it "crystallizes". What is going on is the sugar in "fresh" honey is supercooled below its melting temperature, but with age it sometimes begins to form crystals that are seen easily.

For a more in-depth study you can go to:

https://dspace.mit.edu/handle/1721.1/34440

Vince Calder



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