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See my letter-to-the-editor in "The Physics Teacher", September 1992...
or keep reading below.
/// there was discussion about it in "The
Physics Teacher" in 1992, I published a letter back then. My letter
appeared in the September 1992 issue. To save you the bother of
looking it up, I have pasted the entire letter below.
After my letter was published, some people wrote to tell me it doesn't
work. It does work, but I think I used the wrong words when I
suggested that one "tap the can on the table." One person said it
didn't matter how hard he "banged" the can on the table/// If you
successfully dislodge the bubbles and get them into the head space, you
can safely open the can no matter how hard someone shook it before
giving it to you.
My 1992 letter follows:
* * * *
Dear Sir,
Shaking a can of carbonated soft drink and opening it without making a
mess is indeed an impressive demonstration. It might also show some
important insights about solution chemistry. Unfortunately the proper
insights are not those speculated by Edward Pascuzzi [Phys. Teach. 30,
173 (1992)].
I first saw this when I was a graduate student at Michigan State
University Cyclotron Lab in about 1975. Professor George Bertsch once
appeared at coffee break with a cola bottle on which he had replaced
the cap with a pressure gauge (using rubber tubing and hose clamps).
He made a one dollar bet with all willing that the pressure would not
increase if he would shake the bottle. He gave his word the bottle was
genuine (we could see it had pressure). He had done nothing other than
cap it with a pressure gauge and let it sit on his desk for a while.
George shook the bottle, the pressure did not go up, and he
supplemented his salary with a number of dollars. This of course left
everyone clamoring for an explanation both of why the pressure did not
rise and also why we make a mess if we shake a soda before we open it.
This eventually led to the demonstration that you do not make a mess
if you vigorously tap the can/bottle between shaking it and opening it.
The proper explanation is given below.
If the can has been at its present temperature and pressure for enough
time, the CO2 in solution is in equilibrium with the gaseous CO2 in the
head space. The important chemistry lesson is shaking does not change
this equilibrium ratio. If the system is out of equilibrium shaking
can hasten its arrival at equilibrium, but shaking is not a factor in
determining where that equilibrium is (only temperature and pressure do
that). This statement was proved by noting the pressure did not
increase (i.e. no gas came out of solution) when the bottle was shaken.
Of course this goes against common belief; most people imagine a
pressure increase occurs when you shake the can and this is responsible
for the mess. In reality all shaking does is displace some CO2 from
the head space to bubbles within the liquid and attached to the walls.
These bubbles (not dissolved gas) are the gremlins which cause the
mess. If these bubbles remain attached to the walls when the can is
opened the rapid depressurization causes them to expand (Boyle's Law).
Since they expand beneath the liquid level they expel liquid as they
expand. Tapping the can on the table (or merely flicking it with your
finger a few times) dislodges the bubbles so they can return to the
head space. From the head space the gas harmlessly escapes without
accompanying liquid.
Of course, once depressurized, the equilibrium is upset and CO2 does
come out of solution. This causes the bubbles you see forming in your
glass as you drink the soda. However without further agitation these
come out of solution quite slowly and cause no mess.
* * * * *
Michael D. Edmiston