Minimum Temperature of Supercooled Water
Date: February 2007
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?
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.
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Update: June 2012