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Recently I saw a video in YouTube about how to make ice in seconds.

In this video, salt is added to a glass of water and then a straw is closed from one end and the other side is lit using a lighter and after which it is stirred for 5 seconds and removed.

As soon as it is removed the ice is formed.

What is the chemical reaction that forms ice so quickly?

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The simplest explanation would be that it isn't (pure) water at all. It is most likely a supersaturated solution of sodium acetate - $\ce{CH3COONa}$ (or sometimes sodium thiosulfate - $\ce{Na2S2O3}$) in water.

A supersaturated solution is a solution that contains more solute than normally soluble at those conditions. So as soon as a rough surface is provided for nucleation (formation of crystals), the solute starts crystallizing and comes out of solution as a big crystal.

The rough surface used over here is most probably the rough end of the burnt straw. This produces enough disturbance to cause crystallization. Usually an already prepared crystal of pure sodium acetate is used for nucleation but only if you want high purity crystals.

This is a pretty common trick (also the original video is from trick life -- as mentioned on the youtube description --) and most people who don't know about this phenomenon are usually easily tricked (then again, the "ice" formed would actually be hot since it is an exothermic reaction but you can't feel it through a video right :) ).

See this page and this page if you want more info.

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  • $\begingroup$ Cool! I would never have thought of this explanation, but indeed we don't know that the crystallization that we see is water. $\endgroup$
    – Michiel
    Apr 16, 2013 at 12:52
  • $\begingroup$ Thanks! Actually, I tried doing this for a science fair once but it didn't turn out too well though, I didn't take the right concentration (it's hard to get it right). That's how I came up with this explanation. $\endgroup$
    – kaliaden
    Apr 16, 2013 at 13:28
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I am not sure about this specific video (although it indeed appears fake), but the trick in general is NOT FAKE.

It works by undercooling (also called supercooling) the water, which essentially means that you cool the water (far) below its freezing point but avoid ice formation. A common way to do this is to pressurize the water. This will lower the freezing point so it allows you to cool to say -20$^\circ$C, if you then change the pressure back to normal the freezing will only start when triggered by a disturbance, like a tap, shake or added ice crystals (like in the movie below).

Adding salt is another way of lowering the freezing temperature and achieving a similar result. Also with beer it is possible to achieve undercooling. Just put a closed bottle in the freezer for $\approx 2$ hours then open it and tap the top to see the entire beer freeze up. In this case the freezing occurs because your tapping removes (part of) the dissolved gases from the water thus elevating the freezing point back to normal.

A decent (visual) explanation is given here

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    $\begingroup$ The videao is a fake! Even at best supercooled water will freeze only partly! Second problem is that supercooling water is extreme tedious business, needing extreme lab cleanliness in Water and vessels. Last not least the tiniest mechanical "bump" will start freezing, You would not have a real chance to remove the glass from Your refrigerator. $\endgroup$
    – Georg
    Apr 16, 2013 at 10:32
  • $\begingroup$ @Georg - As I mentioned: I am not sure about the specific video. I AM sure that you can do this trick in general although it is indeed tedious but not at all impossible to try at home. The trick with the beer can work EASILY, because while the cap is still on you are NOT in an undercooled situation yet, only after you remove the cap you get there and then indeed a small tap on the bottle will start the freezing. Please READ what I wrote before you start judging. $\endgroup$
    – Michiel
    Apr 16, 2013 at 11:40
  • $\begingroup$ This answer is really informative.. I didn't know about this phenomenon... and @Georg , michielm has explicitly mentioned that he is not sure about the specific video. The trick is indeed possible by this method but most probably not by the conditions in the video. $\endgroup$
    – kaliaden
    Apr 16, 2013 at 12:34
  • $\begingroup$ No true- supercooling happens all the time- its normal- you mean extreme supercooling like -20C. Its why they use Silver Iodide to cloud seed to suppress supercooling so it freezes at -1C. Some bactaria can get that done to -0.1C A supercooled once seeded freezes completely quickly as the whole lot of water is thermodynamically unstable. $\endgroup$ May 7, 2014 at 1:57

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