TITLE: How does a physical knock initiate freezing of supercooled water?
QUESTION [0 upvotes]: I just stumbled across this YouTube video:
http://youtube.com/watch?v=iihz16t6MHs
What's the mechanism behind it? With a knock, I added some energy. So what?
With a knock, I also increased pressure, but water behaves opposite of most substances (having freezing point raised by pressure) meaning it'd stay in liquid state with that logic.

REPLY [3 votes]: The water is in a supercooled state. That means its temperature is well below freezing (it was put in a freezer for a couple of hours). However, it can stay liquid at that temperature, unless it has impurities that help the formation of ice crystals. Note that the guy used "purified water". Try distilled water that has been boiled to remove any oxygen and other gasses dissolved in it. Very pure water can stay liquid down to $-48\,^{\circ}{\rm C}$. Freezing of water in this state can be caused by even a small shake. Once a single ice crystal forms, suddenly the water has sites to crystallize further, and freezing is almost instantaneous.
In rare circumstances, supercooled water can occur in rain. When that rain hits the ground it instantly freezes, causing the very dangerous "black ice".
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