halle1@houxz.UUCP (J.HALLE) (12/27/83)
All this cold weather allowed me to observe (unintentionally) a phenomenon I learned about in thermo but never actually saw. We keep carbonated beverages in the garage. With the cold spell we've been having, I was afraid it might freeze, but everything looked OK, so I did nothing. Over the weekend I brought in a bottle (2 liter plastic type). When I twisted off the cap, I could watch the ice form in the center, almost instantaneously. This is a fine example of supercooling a liquid and of evaporative cooling.
peters@cubsvax.UUCP (12/28/83)
I doubt whether you observed evaporative cooling, although the soda was
probably supercooled. What probably happened when you twisted off the cap
is that some dissolved CO2 started to escape, and the bubbles formed nuclei
for crystal formation.
There might have been a very small amount of cooling taking place as a result
of the release of CO2 -- but this is not evaporation, which is defined as
the vaporization of a liquid. It is rather a largely entropy-driven reversal
of the solution process; driven, that is, by the tendancy of the CO2 to
expand into a greater free-volume at a lower pressure.
Of course, since CO2 does not form an ideal solution in water,
(i. e., it forms carbonic acid (H2CO3)), there is a small enthalpic
effect, and, in fact, it would have been in the direction of cooling as
bubbles escaped. But I'd "bet my boots" that this effect would have been
insignificant compared with simple nucleation by a bubble or bubbles (remember:
all you need is one!). At the probably low rate of escape of the bubbles at
the low temperature and high viscosity of the solution, the sensible temperaturedecrease due to the escape would have been miniscule and rapidly dissipated
from a local volume.
{philabs,cmcl2!rocky2}!cubsvax!peters
(Peter S. Shenkin, Columbia Univ, Dept of Biol Sci)norskog@fortune.UUCP (01/02/84)
#R:houxz:-62700:fortune:8600006:000:423
fortune!norskog Jan 1 13:03:00 1984
I am confused by cubsvax!peters response. I had assumed that the
phenomenon described occurred because under pressure, the freezing
temperature dropped below ambient temperature, which was slightly below
0 C. Is this in fact what the happened? (Is this what supercooling means?)
Please reply by mail, with polite language,
Lance Norskog
Fortune Systems
{hpda,harpo,sri-unix,amd70,ihnp4,allegra}!fortune!norskog