A number of bottles of pop (two varieties) in unopened plastic bottles have sat for a few years in a room-temperature environment. All of the bottles have now collapsed a certain amount (all to about the same degree).
They all started out under pressure from the carbonation, but now, not only is that gone, but the internal pressure has dropped below ambient. They all still have an "air" space above the liquid contents; not sure if it is or is not more than when originally purchased. If one of these bottles is opened, it draws in air and regains its original shape.
I have the following questions:
What happened to the carbonation? Did it leak out, or did it undergo some sort of chemical reaction, or perhaps both?
How could the pressure inside the bottle drop below ambient? What chemical or physical process is causing it, and what is driving the process? Why would it not simply reach equilibrium with ambient?
What would be the ultimate result if these bottles were left even longer?