I learned something terrific yesterday: if you take a really cold but still liquid beer out of the freezer and open it, the beer will freeze within seconds. The freezing trick also works if instead of opening the beer, you give the unopened bottle a sharp rap. The reasons I've found online for why the trick works varies slightly for the two cases. According to Daryl Taylor's site for science teachers, opening the bottle changes the pressure in the bottle and thus lowers the temperature:
The sealed bottle's envoronment has a specific volume, pressure, and temperature. By changing one, you are necessarily affecting the others. The chilled liquid has a smaller temperature, esentially the same volume, thus a smaller smaler pressure. This is, of cousre, according to the basic gas-law, PVNERT. Better known as PV=nRT. Even though the internal pressure has decreased, it is still far greater than the pressure outside the container, namely one atmosphere. Upon opening, the pressure inside drastically plunges as it tries to equalize with the atmosphere. This rapid decrease in P corresponds to a rapid decrease in T, since the V is essentially the same. This rapid drop in temperature of a liquid that is NEAR freezing actually plunges the liquid into a frozen state.
Not sure I completely buy this...does the ideal gas law work for liquids? I can see that the small amount of gas in the neck of the bottle would decrease in pressure and thus decrease in temperature and that might be enough to spur the liquid into freezing. For a better answer for both cases, I consulted the internet's all-seeing oracle, Ask Metafilter. This comment gives a succinct answer:
The beer is below the freezing temperature, but there is not enough contamination for the ice to form. The bubbles of carbon dioxide released when the bottle is hit act as nuclei for ice crystal growth in the supercooled beer. Same thing happens in reverse when water is microwaved in a smooth container but won't boil until hit.
This more scientific discussion of unfreezable water provides more evidence of what may be going on: supercooling effects, the carbon dioxide in solution hindering freezing (osmotic depression of freezing point), and hydration factors. Anyway, wicked cool! Supercooled beer!
Update: If you require visual proof, check out these two videos of beer freezing after it's been opened. Here's a video with water...so fast! (via digg)
