To Squeeze or Not to Squeeze? (No OtR content)

[Jason Neely <bondibox at bellsouth_net>]

Does allowing room air into a 2 Liter bottle increase the total amount of CO2
released from solution?

Rhine-Folk,

O.K.  I really do pick up on an idea and run with it sometimes, and this is one I
can't let drop.  You've probably by now seen someone (perhaps yourself) squeeze a
plastic soda bottle before capping it.  There are a LOT of people who do it, and I'd
guess 99.99% of them can't even give you solid reason why, other than "it keeps it
from going flat".   Maybe it's the lack of factual justification for their actions
that has me peeved, or maybe that I think they're just plain *wrong*.  I dunno.  I
share this with you all to both allay your anxieties and pique your curiosities.
Truth be told,  I just think you all are intelligent (read: geeky) enough to
appreciate it.

I did ask @ alt.folklore.science and recieved an initial reply to my question
Short answer:  the writer agrees with me, but there is a good reason for others'
misunderstanding (not really, since as I mentioned, most people don't understand why
they do it).

First my argument:

Squeezing the bottle in order to smoothly dispense the beverage, keeping it from
making a "glug" and preventing any agitation of the remaining soda while pouring is
a smart thing.  The part of the process that I have started questioning is after the
pouring is done and the cap is about to be put back on.  There are people who claim
that by squeezing the air out of the bottle before sealing it, that it somehow
prevents the contents from going flat.  I would argue the opposite, that when you
take out that air, it is just that much more gas that the soda will release before
the pressure builds inside the bottle.

I credit the answer to Peter Ceresole, of usenet fame:
It's more a story of partial pressures; the key factor would be the partial
pressure of CO2 in the gas in contact with the soda.  After opening, when you have
mixed air and
CO2 in the top, the thing that counts for the CO2 in solution is not the
total pressure of the free gas, but the pressure *and* the proportion of it
that's CO2.
If the bottle has been squeezed/collapsed, although the pressure is lower- even
initially
negative- the proportion of CO2 is much higher, nearing 100% but sharing
with water vapour of course and will achieve equilibrium at much lower
overall pressure than if you cap it straight away and trap air in there
too. However achieving a positive pressure at all will require deforming
the bottle back to a shape at which is it no longer applying negative
pressure, so there will be a hit on the CO2 to achieve that.


So there's really sound scientific reasons WHY someone would squeeze the bottle
before putting the cap back on, but it really doesn't hold up to practical
applications.  I think the key concept here is that by allowing room air into a 2
Liter bottle you increase the total CO2 pressure needed for equilibrium.  The
pressure of the entire bottle will have to be higher than it would be if it were
just CO2.  So the question is, does the extra air do any harm?  Will more CO2 get
released from solution when creating higher pressure than it would if it had to fill
more volume at a lower pressure?

My hypothesis would go like this, the extra air in the bottle acts as a spacer,
helping increase the pressure the CO2 is under.  Even though it increases the needed
pressure, I contend that the volume it adds is worth it especially if the amount of
liquid is low.  I need formulaic or scientific evidence before I can claim victory
on this point, though I fairly confident in it.  MAYBE there's a point, before the
liquid gets low, where the lower pressure justifies the air lost.  Maybe.**
Secondly, and certainly more important in practical application, is the amount of
gas needed to be released before the sides of the bottle bulge back out and positive
pressure is reached, far exceeds the ability of the soda to restore pressure.  Even
though it's at stasis, the squeezed bottle is for all intents and purposes a vacuum
since it can release gas from its liquid and not change the pressure of its system.
I've seen a lot of still crushed bottles rattling around in refrigerators that never
did reach pressure.  Lastly, and I think this is a good point, when you squeeze air
out of the bottle, some of that air is CO2 which you want to keep inside.

To combine all three points, if you were able to squeeze *just* the foreign gasses
out of the bottle leaving only CO2 behind, the amount of CO2 - lost carbonation -
needed to fill the vacuum and reach equilibrium pressure would be MORE than would be
needed if it had to establish a slightly higher equilibrium of mixed gasses starting
at zero pressure (1 atmosphere).


** Anyone looking for that special science-fair idea?
-jason


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