Re: What makes coca-cola go flat?

Coke like all "soft drinks" is carbonated with carbon dioxide (CO2)a clear, colorless gas. The CO2 is dissolved in the water and sugar mixture under pressure. When the CO2 level just above the liquid is lower in concentration than the CO2 level in the liquid more gas comes out of solution than returns to the solution. The CO2 is lost from the soft drink. When the cap is put onto the bottle the CO2 comes out of solution until the CO2 level above and within the liquid are the same. This is called equilibrium. Shaking a closed bottle of soda will cause the pressure to rise because the CO2 is not in equilibrium anymore. After some time has passed the pressure will drop back to normal because the free CO2 will dissolve back into the liquid, regaining equilibrium. If the soda bottle is half full and the cap is returned to the bottle, the CO2 will come to equilibrium using some of the CO2 from the liquid to do so. The larger the space above the liquid the more CO2 will be required to reach equilibrum. If the cap is left off, the CO2 level will come to equilibrium with the atmospheric CO2 levels (which for this discussion are quite low)and the coke goes flat. If you squeeze the space out of the bottle before you put the cap on, the amount of CO2 needed to come to equilibrum will be less and the drink will keep its fizz. There is one problem with this approach though, when you squeeze the bottle and cap it, you create a partial vacuum. The CO2 level in the space is very low but the pressure will rise when the CO2 comes out of solution. This should inflate the bottle back to its initial shape and volume. Since the partial pressure of CO2 is all that matters, not the pressure of N2 or O2 from the atmosphere, the amount of CO2 removed from the liquid will be the same in either case. If the bottle is constrained so that it can not expand then the squeezed bottle will have more fizz later.