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Re: [Phys-l] two fluids questions

Thanks for the detailed explanation. My chemistry knowledge is somewhat
limited. I was relating the Fizz Keeper to a pressure cooker. Increase the
air pressure and it's harder to boil water. Decrease the pressure (e.g.,
go to Denver) and it's easier to boil water. I thought if you simply
increased the air pressure above a carbonated beverage, the CO2 would
remain in the liquid longer. I guess I was wrong! These Fizz Keepers
should never have been sold!

Isn't there a similar thing for wine bottles? You remove some of the air
from the bottle to reduce oxidation. Anybody out there drink wine? :) I
don't...


Forum for Physics Educators <phys-l@carnot.physics.buffalo.edu> writes:
On 11/18/2010 11:18 AM, Anthony Lapinski wrote:
1. You may have seen a Fizz-Keeper sold in supermarkets. It's a plastic
attachment you screw onto a plastic soda bottle. You pump it to force
air
into the bottle. This creates a higher air pressure above the soda, so
it
should preserve the soda from going flat. Makes sense. However, my
chemistry colleague recently informed me that this does not work! He
said
the partial pressure of CO2 will be the same whether you increase the
air
pressure or simply put the cap on. The best way to preserve the fizz
would
be to add CO2 gas, or simply squeeze the bottle to remove most of the
air
and then cap it. Does this make sense? I know physics, but not as much
chemistry.

The Fizz-Keeper is 98% nonsense.

The story from the chemistry colleague is only about 50% nonsense.

Here's the deal: Dalton's law of partial pressures. The partial
pressure of CO2 is the only thing that matters to the equilibrium
distribution of CO2.

There is one /slight/ way in which the Fizz-Keeper makes a difference,
and that has to do with kinetics, not equilibrium. At higher pressure,
it is harder for CO2 bubbles to nucleate. Result: The soda goes just
as flat, but it takes slightly longer. Also you might be fooled into
thinking you have accomplished something, because you hear the hiss
of escaping gas when you reopen the bottle, and you might associate
that noise with non-flat soda.

Also, at the 1% level or thereabouts, you can dissolve a certain
amount of O2 and N2 into the water. This is useless, but might
allow the Fizz-Keeper marketing department to claim that the device
actually does "something".

Squeezing the bottle to remove the air is nonsense, unless you
plan to keep squeezing it forever. As soon as you let go, the
CO2 pressure will re-expand the bottle and you're back where
you started, only very slightly worse off, because of the
aforementioned kinetic effect.

2. If you shake a plastic bottle of unopened soda, the inside pressure
should remain the same since it is a closed system. But if you then
squeeze the bottle, it certainly feels like the pressure has increased.

Well, that depends on history:
a) If the bottle has recently been opened and re-closed, then
the pressure in the head-space is considerably below the
equilibrium value. Shaking it speeds up the equilibration,
and will produce an easily observable rise in pressure.
b) If the bottle has been sitting around closed for a long
time, the pressure will have equilibrated, and shaking it
will not make any appreciable difference.
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