|MadSci Network: Chemistry|
I can't think of any reason why contact with warm air would make a beer that had been chilled in the freezer freeze, but I can think of an alternative explanation. Many liquids can be cooled well below their freezing points before crystallization begins. This is supercooling, and a liquid below its freezing point is said to be supercooled. A supercooled liquid is not stable and may crystallize spontaneously. Supercooled liquids often crystallize in response to physical agitation of their container, and this is probably what happened to your beer.
This sort of thing happens to me all of the time in the lab. If you wanted to test to see if this is true, take 4-6 beers (you want a couple of data points for the control and experimental groups) and put them in the freezer for roughly the same amount of time that your previous beer was in the freezer. Then take out half very carefully and half with a lot of vibration and motion and see if the latter will freeze without you even opening the bottle. If that works, then very carefully open the tops of the ones that you very carefully took out of the freezer and wait to see if they freeze upon contact with warm air. If not, then you should be convinced that the freezing resulted from agitation of a supercooled liquid and not from contact with warm air.
I read the question and answer to Freezing Beer? and would like to offer an alternate explanation. I forget which of the gas laws it is, that relates temperature to pressure, but I do remember the general form:
P1V1/T1 = P2V2/T2
In the case of the beer, opening the bottle reduces the pressure on the liquid and results in a sudden lowering of temperature--causing the unfrozen liquid to freeze.
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