Home Physics Filter: Does squeezing the bottle of a carbonated beverage help keep it from going flat?
November 3, 2012 7:22 PM   Subscribe

Does squeezing the bottle of a carbonated beverage help keep it from going flat?

My wife does this, and I can not understand how it would be beneficial. Here's a picture of what I'm talking about. How might this keep the gases from escaping? She says it's about having less air inside the bottle. Please esplain to me how this could be.
posted by dammitjim to Home & Garden (12 answers total) 7 users marked this as a favorite
Squeezing the air out keeps the CO2 in solution.
posted by sanka at 7:29 PM on November 3, 2012 [1 favorite]

Squeezing the bottle accomplishes exactly the opposite of what your wife wants to accomplish. Squeezing the bottle creates negative pressure inside the bottle because of the plastic's elasticity that will hasten the extraction of the carbon dioxide from the soda.

That is why the caps that claim to save carbonation increase air pressure instead the bottle, although they do not work as advertised because pumping air into the bottle does not force carbon dioxide back into the soda. You would need to pump carbon dioxide into the bottle instead. The bottles are pressurized with carbon dioxide when bottled, so once you open the cap for the first time, it is a losing battle. The best thing to do is keep the bottle in the fridge. A colder soda will released less CO2.
posted by Tanizaki at 7:34 PM on November 3, 2012 [19 favorites]

I don't think this actually works. I've heard of people doing it before, and so I tried it for awhile, but it seemed to have no effect whatsoever on how quickly the soda went flat. The CO2 just doesn't want to be in solution once it has all that lovely air to mix with. I noticed the few times I did this that after a few hours, the bottle had expanded back to it's original shape. Obviously this could not happen if the CO2 was staying in solution. There's just not enough pressure in the bottle to keep the CO2 in place once the seal is broken, no matter how much you squeeze the bottle.
posted by katyggls at 7:39 PM on November 3, 2012

Tanizaki has it. sanka's answer is untrue and makes no sense.
posted by halogen at 7:46 PM on November 3, 2012 [3 favorites]

Tanizaki is exactly correct.
posted by jferg at 7:49 PM on November 3, 2012

Tanazaki and sanka are both partially correct.

The bottle is only elastic to the degree that it regains its original shape if you squeeze it and then let go (before putting the cap back on). In general, these plastics are rigid enough that they would only partially return to their original shape. So if you collapse it and then put the cap back on, there may be a bit of a "negative pressure" differential between the interior and the exterior of the bottle, but probably not a lot. To the degree that there is, though, that would tend to promote CO2 effervescing back into the air above the liquid until the pressure differential disappeared.

On the other hand, to the extent that you can collapse the bottle and it doesn't regain its original shape, you reduce the volume of air available for the CO2 to effervesce into.

In either case, the partial pressure of the CO2 gas is the same when equilibrium is reached, and the pressure differential between the interior and exterior is zero. So if you have a smaller volume of air in the bottle when this condition is reached, you will retain a little more CO2 in the liquid.

Since the liquid in the bottle pushes outward on the sides, crushing in the sides the bottle tends to be counterproductive. Crushing it downward if possible might be more effective.
posted by perspicio at 7:54 PM on November 3, 2012 [2 favorites]

Squeezing the bottle in the way the picture shows is ridiculous and does nothing. I do it only when it's mostly full, maybe down to half. I squeeze most of the gas out and tighten the cap. No airspace means no space for the CO2 to escape into.
posted by sanka at 7:58 PM on November 3, 2012

Actually, my answer wasn't completely correct either. It can't be true that in each case the partial pressure of the CO2 is the same and the differential pressure between the interior and exterior is zero, since in the crushed elastic container case it would take more CO2 effervescing to equalize the pressures.

> I squeeze most of the gas out and tighten the cap. No airspace means no space for the CO2 to escape into.

sanka, try shaking the bottle after you do that and I think you will find that the CO2 makes its own space to escape into. Over time, though, some will redissolve. But only as long as the interior pressure is greater than the exterior.
posted by perspicio at 8:02 PM on November 3, 2012

It has never worked for me, and I try every single time I buy a 2-litre in hopes that, one day, I'll get it right goddammit.
posted by windykites at 12:16 AM on November 4, 2012

I drink a LOT of soda (always have), and tried this for a little while, thinking the same thing as your wife (less empty space in the bottle for the CO2 to escape to). I stopped doing it after a couple of weeks because it very definitely made the soda go flat much quicker.

Empirical evidence of its efficacy (or lack thereof) would be trivial to obtain. Buy two bottles, empty them both halfway, crumple one, and then close them both for a day. Pour a cup from each the next day and see which is flatter.
posted by Doofus Magoo at 2:01 AM on November 4, 2012 [1 favorite]

The reason fizzy drinks are fizzy is because there is more gas dissolved in them than is sustainable at atmospheric pressure. Any time a fizzy drink is bubbling, it's in the process of going flat. The fact that an unopened bottle of any fizzy drink doesn't bubble inside the bottle shows that there's enough back-pressure exerted by the gas pocket above the liquid to keep all the dissolved gas in place.

That back pressure is much higher than atmospheric. This is easily demonstrated by filling an empty drink bottle with plain water to the same level as an unopened soda bottle, capping it, and squeezing both. The plain water bottle will feel much softer because the air inside it, being at only atmospheric pressure, is much easier to compress.

No airspace means no space for the CO2 to escape into.

This line of reasoning is completely flawed. The only way it could ever work is if the walls of the bottle were strong enough to resist being re-inflated by escaping CO2, which in the case of a typical plastic soda bottle they are absolutely not. There's easily enough excess pressure inside a closed bottle of non-flat fizzy drink to push the bottle walls back out to maximum roundness.

To maximize the amount of gas that stays in solution in a partially used bottle of fizzy drink, carefully decant the unused portion into a smaller bottle that it will mostly fill, and then seal that. Pour gently down the inside wall of the receiving bottle to avoid as much fizzing as possible. For best results use a glass bottle kept empty in the freezer (don't freeze it afterward though, just refrigerate).
posted by flabdablet at 3:01 AM on November 4, 2012 [3 favorites]

It's embarrassingly obvious in retrospect, but flabdablet is completely correct that carbonated beverages have more CO2 in them than is sustainable at atmospheric pressure (holding temperature constant). Therefore, the air above the carbonated beverage in either a collapsed or non-collapsed bottle would eventually achieve a higher pressure than atmospheric.

Bearing that in mind, reducing the volume of air above the beverage does still help from a pressure perspective in some small measure, since any container will resist being re-inflated by escaping CO2 to some degree, provided the initial pressure of the air above the beverage is atmospheric once the cap is put on. The more rigid the walls of the container, the more it helps. So that would seem to argue in favor of collapsing the bottle.

But looking at it from the ideal gas law perspective (pV = nRT), squeezing air out of the container before capping the bottle means decreasing the amount of gas n by reducing the volume V of the bottle. This does not change pressure p because the cap is off. Once the cap is on, n slowly increases because there is excess CO2 in solution, so pV must increase too. But since the container is relatively non-rigid, it is possible to increase V a great deal without doing much to p. In other words, even though some small increase in p will be required to re-inflate the bottle, this will be miniscule compared to the change in V, which means there will be a substantial decrease of CO2 in solution.

So on reflection, I find that even though the reasoning in my earlier answers wasn't altogether wrong, my earlier conclusion, even after amending, was nevertheless incorrect. In fact, I would appreciate if a mod would strike it out and annotate it accordingly to ward against later visitors being persuaded by it.

Now, if you were to limit the potential gas volume by, say, putting a tight strap around the compressed region and/or dropping pebbles in the bottle until it was full, then you'd be in business.

Of course, in the picture provided you'd have to eliminate pretty much all the air capacity to get any kind of useful effect.
posted by perspicio at 8:22 AM on November 4, 2012

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