A friend explained to me that to prevent an opened soda bottle from going flat, the best course of action is to press the bottle before putting the cap back on as to chase air out of the bottle.

This seems counter-intuitive to me, since this should cause the air pressure to decrease inside the bottle (because the bottle tends to snap back to its original form). And it the pressure is decreased in the air around the soda, the soda should go flat faster.

Does chasing air from the soda bottle actually keep the soda from going flat, and most importantly how?

  • $\begingroup$ I'm voting to close this question as off-topic because it’s about food storage and not physics. $\endgroup$ – Kyle Kanos Dec 2 '19 at 15:17
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    $\begingroup$ @KyleKanos How is asking about the physics behind why something works off-topic? $\endgroup$ – usernumber Dec 3 '19 at 9:08
  • $\begingroup$ I've often thought about this myself. My conclusion is that if the bottle is able to snap back you will end up with just the same flatness as if you had left it alone. But if you can put a permanent crimp in it that reduces the volume of air above the soda then you should be better off. $\endgroup$ – user68014 Dec 3 '19 at 9:09
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    $\begingroup$ Well that sounds like a Chemistry question than a physics one $\endgroup$ – Kyle Kanos Dec 3 '19 at 18:08
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    $\begingroup$ Possible duplicate of Why will crushing a partially empty 2-liter bottle keep the soda more fizzy? $\endgroup$ – usernumber Dec 4 '19 at 8:43

My wild guess. According to ideal gas law : $PV=nRT$ in isothermal process if you decrease volume N times, then you need N times less amount of gas to keep same pressure P which was before volume decrease. So in essence, when you squeeze bottle and close it,- carbon dioxide going out to small volume of air quickly restores pre-opening pressure condition of bottle, until it becomes in equilibrium with rest system with no CO2 bubbles coming out. Accordingly, best way to keep the soda from going flat (for longest period and with biggest concentration of $CO_2$) would be to leave no space for carbon gas at all (i.e. so that whole bottle would be filled with carbonated fluid only).

  • $\begingroup$ So even if the total pressure is decreased because of the bottle's tendency to snap back to shape, the partial pressure of CO2 would rapidly be much higher than if the bottle hadn't been squeezed. $\endgroup$ – usernumber Dec 2 '19 at 13:53
  • $\begingroup$ Somehow, I doubt that there is a big pressure decrease due to bottle snapping back to shape. Not all bottle wall's deformations haves tendency to snap back, some of them may be permanent. Besides it depends how you keep bottle in refrigerator - if you will keep it recumbent form - then fluid static pressure on walls will be minimal, and so on pressure decrease too. Of course measurements needed to clarify situation $\endgroup$ – Agnius Vasiliauskas Dec 2 '19 at 14:41
  • $\begingroup$ Keeping the bottle filled at all times would entirely defeat the purpose of having a bottle. Fortunately, there's another way. Keep a supply of CO2 on hand, and always make sure that the space above the liquid is filled with pure CO2 before closing the bottle. $\endgroup$ – Solomon Slow Dec 2 '19 at 14:50
  • $\begingroup$ Indeed, however problem here is "supply of CO2" and "make sure that the space above the liquid is filled with pure CO2". So OP suggestion sometimes may be more easy way $\endgroup$ – Agnius Vasiliauskas Dec 2 '19 at 14:55
  • $\begingroup$ Any reasons of down-vote, of otherwise technically correct explanation ? $\endgroup$ – Agnius Vasiliauskas Dec 2 '19 at 16:48

I don't know the formula, but I'd be willing to bet that when the system is in equilibrium, there must be a mathematical relationship between the concentration of CO2 in the solution, the temperature of the solution, and the partial pressure of CO2 in the space above the solution.

Squeezing air out of the bottle only makes room for more CO2 to evaporate from the solution and re-inflate the bottle before the equilibrium is reached.

  • $\begingroup$ Why more room ? Air density before and after squeeze is the same, just volume changes - it gets smaller. As such- smaller room for CO2 evaporation. Your logic is wrong. $\endgroup$ – Agnius Vasiliauskas Dec 2 '19 at 16:51
  • $\begingroup$ @AgniusVasiliauskas, I think you're making a mistaken assumption. See my comment on your own answer. $\endgroup$ – Solomon Slow Dec 2 '19 at 17:16
  • $\begingroup$ I am not, see my reply above $\endgroup$ – Agnius Vasiliauskas Dec 2 '19 at 17:25

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