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.