Q: Sometimes, when I take a bottle filled with liquid (preferably beer) out of a cold refrigerator, opening it results in the liquid freezing. Why? (continued)
A: While it may be true that pure water in a smooth container can remain liquid well below its normal freezing point (18 February 1995) I doubt that beer is utterly pure, nor is a normal beer bottle all that smooth.
A clean room will make no difference because the bottle is under a higher pressure than the surrounding air and, for some time after it is opened, no particles will be able to enter the liquid and act as seeds for crystallisation.
Advertisement
The real explanation is that the liquid is saturated with CO2 which lowers the freezing point by several degrees. When the bottle is opened the CO2 starts to come out of solution which has two consequences. The first is that the freezing point starts to rise. The second and more important consequence is that lots of small bubbles start to form, which act as seeds. The whole bottle full of beer will turn to foam in an instant.
A good way of observing this is to put a bottle of tonic water, which is usually clear glass or plastic, into a freezer and leave it for a while. When you twist the cap off the bottle you end up with a tonic water ice lolly as soon as the pressure is released. Of course, glass beer bottles left too long in a freezer will burst, leaving a pile of glass-filled beer foam behind.
I have discovered the freezing properties of beer and tonic water without a research grant.
A: Most weeks I am suitably impressed by the erudition of answers given in your column, but the answer to the frozen beer question is, at best, only partly correct. There are actually three processes which will contribute to the phenomenon.
First, bubbles of vapour that are released from solution will act as nuclei for ice crystal growth in a supercooled solution. This is probably the major process at work.
Second, increased pressure lowers the freezing point of water. This is the basis of such commonplace things as why snowballs stick together, why skates glide, and so on. Therefore, lowering the pressure by opening the bottle will let water freeze at a higher temperature.
Third, the gas leaving the beer will expand, so its adiabatic cooling will lower the temperature of the beer.
The nucleation provided by the gas would be more important than anything which might drop in, so opening the bottle in a clean room, as your respondent suggests, would be unlikely to have any effect.