This month's Challenge Question: Specs & Techs from GlobalSpec:
We know that water freezes at 0º C in standard conditions. However, we also know that water can stay in its liquid state well below 0º C in the same conditions; this is called supercooled water. What causes this to happen?
And the answer is:
In order to form ice crystals water needs a nucleating agent. This can be any type of small material (like dust
or dissolved air) which causes the deposition of water molecules, creating the
crystal. The reason has to do with the minimum energy required to start the
creation of the first piece of ice with a minimum given radius so it can grow
to larger radius. If the initial radius is smaller than the minimum critical
radius needed, ice growth will require lots of energy, making the growth too
difficult or impossible. If the initial ice forms on a nucleating agent, growth
will be easier because the initial radius may already be larger than the
critical radius.
Water without nucleating agents can still form ice if the water
molecules meet in certain orientations, but the probability of encountering
this molecular orientation increases if the water is below the freezing point.
Below the freezing point, the water molecules become less mobile and produce
ice crystals more easily.
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