This videos shows that when you shake a soda bottle the pressure does not actually increase. The reason that this occurs is because when the soda bottle is at equilibrium, physical agitation cannot change how much CO2 is in the headspace or the liquid. It remains a constant. This is only true however, if the bottle truly is at equilibrium. In my experiment, to get it to work properly, the bottle had to sit for about 4 hours after I opened it. If the bottle is not at equilibrium then the pressure will likely increase when you shake it because the shaking increases the speed at which the bottle comes to equilibrium.

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