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One of the curses of 2-liter bottles is that when carbonated beverages are stored in them for long periods (i.e. days between drinks) at least the last liter ends up being flat. However, if after every pour the bottle itself is crushed such that the remaining volume of air above the liquid is as small as possible, and such that the structure of the bottle will prevent it from deforming back to its original shape (easily done by crushing the dome first and then working your way down), then the soda will stay nice and fizzy down to pretty much the bitter end, when it becomes impossible to crush the bottle any more but a significant air volume remains.

I know the answer has to do with vapor pressures and equilibrium, but when I try and articulate my thoughts to others their eyes glaze over. Can anyone come up with a broadly accessible, fairly pithy, and yet technically correct explanation?

I realize this is perhaps slightly more on the chemistry side of things than physics, but (1) there's no chemistry Stack Exchange site, and (2) this post seemed reasonable precedence.

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  • $\begingroup$ Nah, this is physics. You're absolutely right--the empty space above the liquid allows gas to build up until it is at an equilibrium with the gas in the soda. By reducing the available space, you further limit the amount of gas that can escape before equilibrium is reached. $\endgroup$
    – Mark C
    Feb 3, 2011 at 5:29
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    $\begingroup$ If there is enough $CO_2$, the pressure inside the bottle would be enough to restore its original shape. $\endgroup$
    – gigacyan
    Feb 3, 2011 at 7:49
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    $\begingroup$ Laws (mechanic, slight vacuum, and Henry's law) are in contradiction to Your observance. Aside the "vacuum" there should be no difference. Or do You keep the bottle compressed by some "squeezing" gadget? $\endgroup$
    – Georg
    Feb 3, 2011 at 14:14
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    $\begingroup$ @Georg Yes, the bottle is kept compressed, but not by a squeezing "gadget" but by the method of squeezing, which uses the structure of the crushed bottle to provide a resistive force against the pressure inside the bottle. If the bottle is crushed to leave very little air volume then the pressure is only able to act on the tiny surface area in the throat, not enough to re-inflate the bottle. Good clarification. $\endgroup$
    – Adam Wuerl
    Feb 3, 2011 at 21:05
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    $\begingroup$ I hate to "draw the worms out of someones nose" (German wording if someone releases information in small bits only and only when asked for). The plastic bottles I know, would "decrumble" on release. $\endgroup$
    – Georg
    Feb 4, 2011 at 11:00

5 Answers 5

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I started writing a catchy but long explanation, but since you understand it and want something short for others, let's try this...

If you put too much salt in a glass of water, you saturate the water and end up with salt sitting at the bottom of the glass. If the temperature changes, the amount of salt that can dissolve changes (more for higher temperature, but you can leave that out). (For extra pithy-ness, leave this paragraph out entirely.)

For gases, besides liquid temperature, gas pressure matters. More pressure means more dissolves. When you open the soda and lose the factory-provided pressure, the gas pressure above the soda is suddenly lower, so carbon dioxide starts leaving the soda. It keeps doing this until "enough" CO2 is in the space above the soda. More space means you need more CO2 to fill it up. So, if you crush the bottle to leave less space, less CO2 escapes from the soda, and it stays fizzy.

Of course, this glosses over a lot of usefully clarifying stuff, such as the concepts you mentioned in your post, but it keeps it short. If you can hold their attention long enough, I would throw in a comment about how only the CO2 pressure matters, not the general gas pressure, just so they don't buy those worthless "pump air into your soda bottle" devices.

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    $\begingroup$ LOL at "those worthless 'pump air into your soda bottle' devices". I suppose those would work great...on Mars. Except even there atmospheric pressure is so low and the cost to transport soda to the surface so high I suppose you have other problems. $\endgroup$
    – Adam Wuerl
    Feb 3, 2011 at 21:08
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    $\begingroup$ Won't the crushed bottle simply expand as pressure increases, increasing the amount of space available and therefore further draining the soda of its dissolved CO2? $\endgroup$
    – Ajedi32
    Jan 1, 2016 at 17:55
  • $\begingroup$ Here's how I'd explain it: There's a lot of CO2 in the soda and it wants to get out. When the soda is new, there's very little space available, so the CO2 starts getting out and starts building pressure. Eventually there's so much pressure that the CO2 in the soda can no longer leave it. When your pour some soda, there's more room available it'll take even more CO2 for the pressure to stabilize. When you crush the bottle then there's less room and less CO2 will leave the soda until it stabilizes. And hopefully the pressure that builds up isn't strong enough to de-crush the bottle. $\endgroup$
    – HSNN
    Aug 6, 2022 at 18:17
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The bottle would tend to return to its original shape. Crushing a two-liter bottle would provide room for expansion, which would allow for more carbination to leave the soda in order to fill the expanding volume to its maximum pressure. By leaving the bottle uncrushed, you remove the expansion factor, thus reducing the loss of CO2 in order to reach the same maximum pressure.

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    $\begingroup$ Anyone care to explain why this is downvoted? It seems correct to me. $\endgroup$
    – Ajedi32
    Jan 1, 2016 at 17:57
  • $\begingroup$ Actually, that'd only happen if the amount of pressure needed to uncrush the bottle was lower than the pressure built up by the CO2 that left the soda. How you crush the bottle matters. You can't just squeeze it, you need to twist and turn until it is able to hold the new shape even if the cap is off. If you just squeeze then bottle will return to its original shape as the CO2 leaves the soda. $\endgroup$
    – HSNN
    Aug 6, 2022 at 18:49
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The question comes with the assumption that we believe the accompanying "factual" information which explains how soda supposedly stays fresh if the air is squeezed out of the bottle after each pour.

I would like to see a comparison at least because I find it difficult to believe. When I crush a large soda bottle it immediately tries to pop back out as far as it can. I think that if you crush it and cap it you'll make the situation worse since the tendency for the plastic to spring back to its original shape will cause a vacuum thereby pulling more CO2 out of solution than if it had not been crushed.

If the plastic is allowed to snap back as far as it can before capping the bottle then there's still a great deal of flexing that can happen and the CO2 will escape and begin to fill the bottle, pushing outward on all the wrinkles until the bottle is back to its old shape.

BUT, if air is pumped into the bottle to raise the PSI then less CO2 will be liberated since it will have no place to go. The area inside the bottle would already be mostly occupied by compressed air.

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Two things affect the solubility of the gas in the liquid. Higher pressure and lower temperature will increase solubility. Without the tensile stresses from the bloated bottle, there are no reactionary forces to resist the expansion of the gas.

Since gases are soluble in each other too, i.e. $\mathrm{CO_2}$ and air mixture, you must be concerned with partial pressure, which is the pressure that an individual gas contributes to a system. The total pressure is the sum of all partial pressures.

You can determine the volume of air in the top of an open soda bottle, and you know it is at atmospheric pressure. Any additional pressure is from the $\mathrm{CO_2}$ coming out of solution with the liquid. If you read the gauge pressure, that would be a fairly good estimate of the partial pressure of $\mathrm{CO_2}$ in the bottle.

Gasses are more compressible than liquids, so if there is more gas to begin with, air mixture or $\mathrm{CO_2}$, the liquid will have to give up a larger mass of $\mathrm{CO_2}$ to raise the pressure in the container to the vapor pressure of $\mathrm{CO_2}$. I've actually thought about this a lot, trying to imagine a rigid container with an adjustable volume, that is also airtight.

Another thing to consider is that putting anything in solution with water will raise the boiling point and lower the freezing point. Here's a fun experiment. Put a bottle of seltzer in the freezer until it gets really cold. Almost frozen. Take it out an open it up. When the $\mathrm{CO_2}$ come out of solution with the water the freezing point will rise back above the temp of the liquid and it will freeze almost instantly.

I can't really think of a simple explanation, but maybe talking about dissolved solids will help people understand dissolved gases. And make gas properties clear before you give this explanation.

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If you left your cola out in the open it would get very flat because all the CO2 would leave it and blow away. Putting it in a bottle with no air it stays fizzy. Having it in a bottle with some air is between these two extremes.

It doesn't much CO2 when its shipped in a filled-up bottle because there are two activities going on. CO2 leaving the drink for the air, and CO2 going back into the drink from the air. Over a long time (probably a matter of hours) these processes have to balance, and from then on you'll not lose any more CO2. When they are in balance it will amount to a certain percentage of CO2 in the air and a (different) percentage of CO2 in the water. For example, there might be 1g of CO2 in each cubic centimeter of air (I don't know, but could look the number up), while there's 2g of CO2 in each cubic centimeter of drink.

There are two effects. First, some of the CO2 gets out of the drink and goes into the air in the bottle. The more air the more CO2 you lose this way. Second, each time you open the bottle the same thing happens again.

It takes a while for your drink to become flat because it starts with quite a lot of CO2. To get an idea of how much CO2 is in your bottle, look for the youtube videos showing what happens when you drop certain candies into it:

CO2 erruption from wikipedia

http://en.wikipedia.org/wiki/Soda_and_candy_eruption

That's a lot of CO2, but eventually, if you open up that bottle enough, most of it will get out.

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