Why will crushing a partially empty 2-liter bottle keep the soda more fizzy?

Question

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.

Answer 1

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.

Answer 2

But by crushing the bottle aren't you creating a negative pressure inside the bottle that's just going to suck more CO2 out? The bottle wants to return to its former larger volume..

Answer 3

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.

Answer 4

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:

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.