1. I suspect your chemistry colleague is simply wrong. Taken to the extreme, his
argument that the CO2's partial pressure won't change would indicate the same
result whether there's a vacuum above the fizzy drink, or a pure CO2 atmosphere.
The suggestion to preserve the fizz by removing the air is absolutely tosh.
2. The pressure *does* go up; by changing the pressures in parts of the bottle,
one allows CO2 to come out of solution, which of course increases the volume it
takes up. But, given time (and room temperature), the equilibrium is restored,
as you posit. It may not be quite back at the starting point (same concentration
of gases in the fluid; same head pressure, etc.) but it will be pretty close.
/**************************************
"The four points of the compass be logic, knowledge, wisdom and the unknown.
Some do bow in that final direction. Others advance upon it. To bow before the
one is to lose sight of the three. I may submit to the unknown, but never to the
unknowable." ~~Roger Zelazny, in "Lord of Light"
***************************************/
________________________________
From: Anthony Lapinski <Anthony_Lapinski@pds.org>
To: phys-l@carnot.physics.buffalo.edu
Sent: Thu, November 18, 2010 11:18:09 AM
Subject: [Phys-l] two fluids questions
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.
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. Is
this a temporary thing until equilibrium is established after some time?