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A friend insists that, to preserve the fizz in an open can of soda, you just put it in the refrigerator inside an unsealed plastic bag. The theory is not that the bag preserves the pressure in the can, but that the $CO_2$ initially outgassed by the soda displaces the air in the bag. This increases the partial pressure of the $CO_2$ above the surface of the soda, thus slowing down further outgassing of $CO_2$.

Does this make sense?

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  • $\begingroup$ You say an unsealed plastic bag? I don't see how an unsealed plastic bag would help. If you are relying on the carbon dioxide not to escape because it is heavier than air, then the walls of the can should be sufficient. $\endgroup$ – Brian Moths Sep 24 '15 at 22:37
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This does not work because your friend's theory is wrong, even though it is based on a sound principle.

What he claims would be true if the $CO_2$ concentration in the can was indeed in equilibrium with the partial $CO_2$ pressure of the air in the bag but that's not the case.

A typical, unopened pop bottle is at about $4$ to $5\text{ atm}$ of pressure. Opened carefully (and without excessive shaking) the pressure drops immediately to atmospheric pressure but the $CO_2$ content remains much above the solubility limit in water, which is why the bottle continues to slowly emit $CO_2$. Even if you cap the bottle again $CO_2$ continues to escape from the water, despite the partial $CO_2$ pressure of the air in the bottle now rising sharply.

The best, but still quite ineffectual method to contain the $CO_2$ as much as possible is to recap the bottle.

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