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Path: utzoo!linus!philabs!cmcl2!rocky2!cubsvax!peters
From: peters@cubsvax.UUCP
Newsgroups: net.physics
Subject: Re: Interesting phenomenon observed
Message-ID: <130@cubsvax.UUCP>
Date: Wed, 28-Dec-83 15:02:02 EST
Article-I.D.: cubsvax.130
Posted: Wed Dec 28 15:02:02 1983
Date-Received: Fri, 30-Dec-83 02:19:48 EST
References: <627@houxz.UUCP>
Organization: Columbia Univ Biology, New York City
Lines: 21

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)