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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.

RedGrittyBrick
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Earth is a Spoon
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    Note that questions that depend on a user clicking a link to see figure out what you are even talking about do not make for good questions. We prefer our questions to be self-contained, with links provided for reference & not necessity. – Kyle Kanos Dec 21 '15 at 11:09

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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".

hdhondt
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  • In addition, the spring-back from the knock could have caused a cavitation bubble, which would have also created the first nucleation site. – matscienceman Dec 21 '15 at 10:35
  • Your answer didn't address the effect of the knock. – Earth is a Spoon Dec 21 '15 at 11:04
  • Well to be fair it basically did. The supercooled state is unstable, meaning that a small crystalline-perturbation (impurity) will grow leading to an ice-block. What the knock does is create this initial perturbation (either directly by rearranging water molecules or more likely in terms of bubbles see @matscienceman ) – Bort Dec 21 '15 at 13:20
  • The "black ice" comment is off base. Rain drops form on small dust particles that are good nucleation sites. Supercooling of raindrops is unlikely. Black ice forms when rain drops hit already frozen ground and freeze quickly. – Lewis Miller Dec 21 '15 at 14:28
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    @LewisMiller According to Wikipedia "if the subfreezing layer of air at the surface is very shallow, the rain drops falling through it will not have time to freeze and they will hit the ground as supercooled rain". – Keith McClary Dec 22 '15 at 02:46
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    I stand corrected. Also, the temporal aspect is not the only mechanism at work here. The surface tension for small droplets significantly increases the pressure inside the droplet which lowers the freezing point. – Lewis Miller Dec 23 '15 at 03:36