But according to Henry’s law, pressure isn’t pressure—it is the partial pressure of each individual gas in the liquid and gas above it that matters. According to Henry’s law, the solubility of any given gas in a liquid is:
where S_{\rm gas} is the concentration of gas in the liquid, K is the Henry’s law constant for the solubility of that specific gas and liquid pair, and P_{\rm gas} is the partial pressure of the gas above the liquid. Hence, it doesn’t matter what the pressure of the nitrogen and oxygen pumped into the bottle may be, the carbon dioxide dissolved in the pop will continue to come out of solution until the partial pressure of CO₂ in the gas rises to the equilibrium point with that dissolved in the liquid.
The solubility K of different gases in water (which I’ll assume is the same as the liquid in the bottle) varies widely, so the behaviour of the gases involved is very different. Here is the solubility of the three main gases we’re dealing with here, all for 5° C, the temperature of a typical refrigerator, and all in units of grams of gas per kilogram of water.
- Nitrogen 0.275
- Oxygen 0.65
- Carbon dioxide 2.9
So CO₂ is 10.5 times as soluble in water as nitrogen and 4.4 times as soluble as oxygen. Consequently, after the bottle is pressurised with air, the main flow of gas will still be CO₂ from the liquid into the gas above it. Nitrogen and oxygen will dissolve into the liquid, but since their solubility is so much lower, a far smaller volume than the flow of CO₂ in the opposite direction. As a result, the CO₂ coming out of solution will add to the pressure of the nitrogen and oxygen in the bottle, raising the pressure and resulting in the “explosion” when the Fizz-Keeper was removed at the end.