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Originally Posted by Pip
But the 1/4 bottle was completely frozen while the unopened one was still completely liquid, so I don't think that could be it. I've seen the same thing happen with soda bottles left out in a truck overnight, if there was one that had been opened, no matter how much or little there was in it, they'd be frozen, but the unopened ones still looked okay. Until you opened them, that is, then they'd turn into instant slush. My ChemE friends would say this was because the soda had been 'under-cooled' (not sure of the english term) so that the fluid temperature was below freezing, and when the bubbles started stirring the fluid that triggered the freezing process. Kind of like when you have under-cooled rain, when it hits the ground it instantly turns to ice.
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I was a ChemE undergrad... Under-cooling with any sort of stability generally only occurs with very slight drops below the freezing temperature and the medium must be totally unagitated. Otherwise the cooling must occur very quickly which was not the case. Also, this is probably not the phenomena that would be observed in the case of a carbonated beverage. What would more likely be happening is that the one that was opened was largely decarbonated. Since water (and most liquids) decrese in freezing temperature and increase in boiling point when there is dissolved solute (which is why we put ice on sidewalks), it is actually the dissolved carbon dioxide that would really be causing the other not to freeze, which also can explain why it instantly flurries with ice upon opening.
Don't underestimate the low thermal conductivity of a plastic bottle. Especially since the other one was open. The cold air can get in the open bottle, cool off and freeze the water which in turn heats the air, which rises and escapes the bottle, allowing cooler air to again touch the surface. This movement of heat caused by a flow of molecules (convection) is generally faster than heat transfer by collisions between molecules (conduction). The exception would be convection with a highly insulating material or conduction with a superconductor, and neither of these are part of the scenario.