(4) Why CO[sub]2[/sub] ?
CO[sub]2[/sub] is a good choice for making fizzy drinks, because not only does it dissolve in a liquid, like other gases, but also reacts with water to form carbonic acid, and its ions. Thus the equilibrium concentration of CO[sub]2[/sub] in the air ,vs solution, lies towards more total carbonates in solution than with a ideal, nonreactive gas. If you charged a bottle of Soda with helium, rather than CO[sub]2[/sub], most of the gas would come out in one great bump immediately upon opening. However, the kinetics of CO[sub]2[/sub] <->HCO[sub]3[/sub][sup]-[/sup] <->CO[sub]3[/sub][sup]2-[/sup] are slow (~seconds), so not only do you end up with a lot more potential gas dissolved when you use CO[sub]2[/sub], but it comes out more slowly.
The CO[sub]2[/sub] <->HCO[sub]3[/sub][sup]-[/sup] <->CO[sub]3[/sub][sup]2-[/sup] equilibrium depends on temperature, pH, and yes, pressure. Data on the pressure effect is hard to come by, however before the last thread on this subject crashed-and-burned, I did find a site that gave a few figures. Can’t locate it now, but it’s a small, nonzero, effect.
With plastic bottles at least, there may be a simple explanation for why some the pump seems to work for some people:
The soda pop is supersaturated with respect to CO[sub]2[/sub] (all those carbonates…).
The bottle is flexible.
When you close the bottle you tend to squeeze it.
This results in a low initial air-pressure inside the bottle, and walls that are easy for increased pressure to deflect.
Since the pop is supersaturated, carbon dioxide gas will come out of solution, until the point where the walls of the container supply enough rigidity for pressure to build and stop the process.
Using a pump lets you use air, rather than fizz to make the container rigid.
If that’s not too clear, think about what would happen if you quickly pulled a vacuum on a soda bottle, and then sealed it. You’d end up with a normally inflated bottle with a pressurized atmosphere of nearly pure CO[sub]2[/sub] above the pop. - Flat soda.