I always thought it sounded like crap. 
Once the CO2 has come out of the liquid, unless you have some sort of device to do it specially, you can’t just make it go back in by doing something to the bottle like squeezing it. It’s put in the liquid under high pressure. We used to have a machine for making pop at home-- it would carbonate the water and then you’d add syrup to flavour it.
And yeah, squeezing the bottle and capping it while squeezed makes a lower pressure in the bottle’s airspace, so more gas would have to come out of the pop to equalize the pressure again. CO2 would stay in the bottle, but it would not be in the pop.
Okay, so what if you squeeze the bottle to get rid of the air space, and then wrap it tightly in tape so that it can’t the escaping CO[sub]2[/sub] can’t reinflate the bottle? Or maybe if the tape doesn’t do the job, you could dip the crumpled bottle in wet concrete and let it set … 
I still don’t think it would force any CO2 that was in there back into the liquid. Sure, it would keep it from expanding the bottle further. There’d still be an equilibrium point where the amount of CO2 getting into the liquid equals the amount escaping.
To recap and rephrase what others have correctly said:
(Grrr… it didn’t post when it’d have been original!)
The CO2 in the cola reaches an equilibrium with the gaseous CO2 when the partial pressure of gaseous CO2 in the bottle is sufficient to to force CO2 to dissolve as fast as it is released (by the definition of equilibrium)
Let’s assume a 2 L bottle, carefully stored at 0 C, that has had 1 L of cola carefully decanted from it before closing.
The CO2 partial pressure is greater than 1 atm. We can feel the pressure in the bottle and hear a a volume of gas escape that clearly exceeds the modest effervescence of our carefully stored and opened bottle.
If we squeeze all the gas out of the half-empty 2L bottle, the CO2 partial pressure is exactly zero. Once the first bubble forms the pressure will initially be less than 1 atm (what some have incorrectly termed a ‘negative pressure’), due to the recoil force of the sides of the bottle. However, this is irrelevant because at equilibrium, the bottle will return to 2L at >1 atm, whether it expands to that size or is capped at that size. We only care about the terminal state.
**The correct comparison is between the total CO2 in the system:
A. (squeezed bottle) = dissolved gas contained in 1 L cola
B. (unsqueezed bottle) = dissolved gas contained in 1 L cola PLUS 1 liter of mostly CO2
However, since the solubility of CO2 in water is roughly 300 volumes at STP (O C, 1 atm) and greater at higher pressures, the difference is less than 1/3%. [There are over 300 liters of C02 at (0 C, 1 atm) dissolved in the cola, and no more than 1 L in the gas above the cola.]**
In short, you will argue this point forever, and neither will detect a difference that prove them right without lab testing. It makes little difference unless you leave the bottle open so long while you argue that it’s mostly flat when you cap it.
However, (I was surprised to realize) the cola may benefit slightly in the very long term from having oxygen excluded. Oxidation and acidic effects do affect the taste of cola over time. That’s why there’s a “Best by” date on cola. Again, not a difference that will be noticed in normal use.
Woah…is this right? This changes things quite a bit.
The reason I ask is, if we assume that the co2 in liquid and gas do not come to an equilibrium immediately upon opening (which we know) and that the pressure in the bottle each time comes to equilibrium at 2 atm. (obviously wrong, but just for quick calculations) and that we start with 300L of co2 at stp, then each time you opened the bottle, the 1 liter of co2 at 2 atm becomes 2 liters at 1 atmosphere (due to decompression), one escaping the bottle, one remaining in the 1L void in the bottle. Recapped, it pressurizes to 2 atm again. Repeat until flat. According to this, this cycle can be repeated 300 times. Crappy mathematics, but it gets us into the ball park.
I only get about 5 cycles at home.
Do these even work? I have my suspicions.
Sure, if you increased the CO2 PRESSURE in the bottle, that would help a lot. But does increasing the AIR PRESSURE do anything to slow down the loss in dissolved CO2?
If the partial pressure of CO2 is zero, it doesn’t matter how much air you pump into the bottle: the CO2 partial pressure remains zero.
It’s easy to preserve the carbonation.
Just go down to the supply room in the basement of the Chemistry building on campus, ask for a small chip of dry ice, drop it into the bottle, and cap it tightly. Heck, this might even restore the carbonation to levels HIGHER than it had been in the store.
And for that amount of dry ice, they won’t even bother to fill out paperwork to charge it to your account.
Just be very careful when opening the bottle later on.
Dry ice is not “food grade” as far as I know, it may contain oil from the manufacturing process. It will likely cause the soda container to explode as well. Very impressive “bang” though.
No, of course it doesn’t. Get an empty bottle and crumple it. It doesn’t bounce back automatically. Blow in it a bit… it still doesn’t rebound. You need to increase the pressure significantly before the bottle starts to deform back to it’s original shape. Assuming that the terminal state is going to be 2L is the flaw in your logic.
I came in here feeling all smart and ready to say “Squeeze the bottle! Squeeze the bottle!” Now I just feel stupid.
Thanks guys,…
sorry for the highjack
This sort of thing is now illegal in many communities. Here’s an example from california.
Well, the results are in and I dod suspect I win :D. This morning I stick a 2L bottle of freshly opened Cream Soda in the fridge squeezed. By nightfall, the bottle was exactly the same shape that I had left it in. Thus, confirming my theory. Given Cream Soda isn’t coke but since its more carbonated, the results are still valid.
It seems the final answer is that squeezing DOES work.
If the bottle doesn’t return to its previous shape, then the net result is less airspace to be filled with CO2, and thus less CO2 lost compared to not squeezing.
If the bottle DOES return to its previous shape, the result is the same amount of space to fill with less gas already there, resulting in a greater loss of CO2.
Pumps increase the air pressure in the bottle, which decreases the volume of CO2 required to get the system up to equilibrium.
But if you finish the damn thing the first time you open it, you won’t have to worry about it.
No, mr Idiot. I suggest you read up on the subject of partial pressure.
Read KPs post again, it sums up the issue fairly well.
When I read this, I thought it was time for you to put down the cheap booze. Take an empty bottle, deform it, presssurize it with your lungs/mouth. You can return it to its original shape. Then squeeze the bottle a little while still pressurizing. You can feel the pressure. Next feel an unopened coke bottle at the store. Very highly pressurized. That is at equilibrium, as it has not been opened for some time. You think that amount of pressure won’t reform a squeezed bottle?
But then you wrote this:
My contention was that the bottle WILL spring back, and that is worse than not having squeezed it.
But yours didn’t spring back to its origninal shape. Why not?
Two possibilities:
-
MORE LIKELY. The bottle had not yet come to equilibrium. Especially if you had it cold, it was still releasing co2 into the bottle. If you’d left it for longer, like a week, it would pressurize more and reform.
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LESS LIKELY. The amount of co2 released reduced the pp required for equilibrium to less than what will reform the bottle.
I’m guessing it’s number 1). So, what does it mean?
What it means is that the RATE of co2 release is more of a factor in common use than EQUILIBRIUM, since equilibrium is not met overnight after an opening. Keeping the rate down is important (i.e. keeping the bottle cold, not disturbing the liquid).
As far as wether to squeeze the bottle or not, go ahead and squeeze. If you find it the next time still squeezed, you’ve lost nothing (and possibly gained by slowing the rate due to a slightly higher pp). If you find your bottles reformed, even a little, don’t squeeze them anymore as this disturbs the liquid and creates more void for the co2 to fill.
See? I changed my mind.
That article states it is illegal to build a dry ice bomb. Presumably, that would only apply to an intentional effort to cause an explosion. I’m not sure you’d get in trouble for putting a calculated amount in the bottle, keeping the size of the dry ice chip small enough to preclude the possibility of explosion. Of course, I’d have to see the actual statute to be sure.
glilly: I don’t think you’ve actually played around with an empty coke bottle. I actually have one here with me as I type so I can finally put this to rest.
Okay. Squeeze an open, empty coke bottle and let it rest. Clearly, the bottle does not deform back to it’s original shape. This means that the plastic of the bottle is strong enough to oppose 1atm worth of pressure (which is really 0atm worth of relative pressure since theres also 1atm pushing from the outside but we’ll ignore that).
I also happened to have that still half-full bottle so I did a second test. Squeezing all the air out and capped, I proceeded to shake the bottle. This DID produce enough pressure to make the bottle return to it’s original shape. However, its clear that the pressure of a shaken bottle is significantly higher than one merely at equilibrium.
So we’ve established the parameters, a bottle will return to normal at something between 1 atm and the pressure of a shook bottle. The question is where on that line it lies.
Now, your assertion was that I did not allow enough time for the bottle to reach equilibrium again. Fine, once I judge the shaken bottle to be safe enough to open, Ill resqueeze it and leave it in there for as long as I can… actually, scratch that, I’ll go out tomorrow and buy a brand new bottle so that I can finally put this damn thing at ease. All for the sake of science eh glilly
However, regardless of the equilibrium pressure, I HAVE proven that any bottle left for less than 30 hrs would be better with the bottle squeezed than unsqueezed. Thus, usage also seems to play another factor in the squeezed/unsqueezed debate.
So far, we have ascertained that if you are the type of person who drinks soft drinks daily, then squeezed is most certainly better. For longer periods of time, we have no current data.
Upon further experimentation, it seems that the shape you squeeze the bottle into has a significant effect on how much pressure is required to return it to normal. As always, this is yet another factor that has significant ramifications. It seems squeezing it flat allows to flex slightly under a very light pressure but takes significant pressure for any large amount of flexing. Wheras more rounded squeezing makes it far more rigid under light pressure but returns back to normal at pressures far below the flat squeeze. So it seems that its impossible to find the optimal squeeze as well without jet more experimentation 
aha! I have a cunning plan…
If I get two identical bottles and squeeze them both, I can leave one unshaken and one shaken. Then, I will have one of them releasing CO2 into the bottle while the other is absorbing CO2. Thus, I will be able to tell when equilibrium is reached since both will have the same pressure. Ill modify the experiment to take this into account as well.
I didn’t mean to make it sound like you hadn’t left it along for long enough, but instead, you DID leave it for the right amount of time (for your average coke drinker), and that not reaching equilibruim may be an explanation as to why the bottle did not reform. I mean, hey, we’ve got busy lives. If thermodynamics can’t keep up, it’s just going to get left behind!
But I may have been wrong again:
I’d always thought that shaking the bottle would just make it return to equilibrium faster, like dropping a crystal into supercooled liquid, but maybe it does actually force the gas/liquid to the other side of equilibrium – too much gas pp, not enough dissolved.
I’ve been trying to figure out how to attach a pressure guage to a 2-liter. Then we could do all kinds of experiments. How long does a bottle take to come to equilibrium? Does shaking a bottle at eq pressurize it?
But I suppose it’s all academic at this point, as you’ve demonstrated that under normal drinking conditions, a squeezed bottle does not reform. So, do you agree with my assesment that squeezing’s OK unless, for whatever reasons, the coke drinker finds his bottles reformed, in which case don’t squeeze anymore?
Let me know how the experiments go. How will you know if the pressures are equal?
And how much does a 2-L cost in HK?