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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?

usernumber
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  • I'm voting to close this question as off-topic because it’s about food storage and not physics. – Kyle Kanos Dec 02 '19 at 15:17
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    @KyleKanos How is asking about the physics behind why something works off-topic? – usernumber Dec 03 '19 at 09:08
  • 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. – user68014 Dec 03 '19 at 09:09
  • @usernumber because “how does x work “ is almost surely an engineering question. Asking “what’s the physics behind x” is extremely vague and hence, too broad to be interpreted in a single way to find the single one answer that is expected in the SE platform. Lastly, your interest seems to be keeping soda (i.e., food) from going bad, rather than anything physics, which should be asked on [cooking.se] instead – Kyle Kanos Dec 03 '19 at 11:42
  • Hence, this question is off-topic here (the same argument would apply to the question that seemed to spawn this one, about storing sodas horizontally vs vertically) – Kyle Kanos Dec 03 '19 at 11:44
  • @KyleKanos My interest is why squishing the bottle would keep the carbonated gas dissolved in the liquid even though the tendency of the bottle snapping back should decrease the pressure in the bottle, so I don't think the question is on-topic for Seasoned Advice. – usernumber Dec 03 '19 at 14:50
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    Well that sounds like a [chemistry.se] question than a physics one – Kyle Kanos Dec 03 '19 at 18:08
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    @KyleKanos you seem to have the same idea of chemistry as Dirac. I find it too narrow. Do you really think there is any chemical reaction in a soda bootle? Come on, it is genuine physics. Maybe you may want to call it chemical physics. But still physics. Principls of everyday life physics are well inside the perimeter of SE.physics. – GiorgioP-DoomsdayClockIsAt-90 Dec 08 '19 at 04:57

2 Answers2

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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).

Agnius Vasiliauskas
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  • 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. – usernumber Dec 02 '19 at 13:53
  • 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 – Agnius Vasiliauskas Dec 02 '19 at 14:41
  • 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. – Solomon Slow Dec 02 '19 at 14:50
  • 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 – Agnius Vasiliauskas Dec 02 '19 at 14:55
  • Any reasons of down-vote, of otherwise technically correct explanation ? – Agnius Vasiliauskas Dec 02 '19 at 16:48
  • I was not the down-voter, but you seem to assume that after the bottle has been squeezed to a smaller size, that it will rigidly stay in that same configuration even as the internal pressure increases. That's not my experience of how soda bottles behave. If you don't put a clamp on it, the bottle will expand again to almost its original shape before any significant pressure is able to build up inside. – Solomon Slow Dec 02 '19 at 17:16
  • See this keeping same bottle configuration is probably one condition of this method. So YES, it will stay. Maybe you just need to press harder on bottle or something for changes stay permanent and also there must be enough of free space with air. – Agnius Vasiliauskas Dec 02 '19 at 17:19
  • Also it may depend on bottle type. Some may be more "squeezable" than others - for example, metal can has more "permanent type" deformations than plastic ones. – Agnius Vasiliauskas Dec 02 '19 at 17:35
  • Metal soda cans usually are not resealable, so moot point there. As for the YouTube link, Did you want me to believe the video? or did you want me to believe the comments, every single one of which said, "This is stupid," or "This is wrong." I'm going to side with the commenters. Yeah, the video shows the bottle coming out of the fridge, still in the same squished shape, and it shows the soda still fizzy, but there's no indication of how many seconds elapsed between putting the bottle in to the fridge and taking it back out again. I'm gessing that the video producer did not wait overnight. – Solomon Slow Dec 02 '19 at 18:02
  • P.S., Anybody who wants to know the real answer can simply try the experiment at home. – Solomon Slow Dec 02 '19 at 18:08
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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.

Solomon Slow
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