I'm basing this off of a home experiment I did to demonstrate Torricelli's law.
If I take a regular plastic water bottle and poke a small hole at the bottom, water will not flow out. This was because the bottle cap was still on and thus any small changes will cause the pressure inside to not be equal to the pressure outside and suction will occur. If I take off the cap, water starts flowing out with a speed $\sqrt{2gh}$. If I put on the cap again, water flow stops. This is all fairly standard.
I then tried to conduct this experiment but by pouring soda into a plastic bottle. I recently read that shaking a soda bottle does not increase the air pressure inside. However, when I shook the bottle and then poked a hole (with the cap still on), soda started to flow out! When I took off the cap, the motion of the soda surprisingly stopped! (I'm guessing this second part was due to surface tension taking effect due to a low bond number). When I put the lid back on, nothing happened.
The logical conclusion I made from this was that shaking a bottle does indeed change the air pressure inside (perhaps by a tiny amount). When the cap was taken off, the pressure inside was equal to the atmospheric pressure which was why nothing changed if when I put the cap back on.
Is this correct? Or is there something else I'm missing here?
Best Answer
Shaking the soda bottle makes the dissolved gas escape the liquid. As long as the lid is on, this can't happen (or happens for a little bit and then stops): the air in the bottle is already saturated with gas.
Then you poke a hole in the bottle, below the surface. The liquid may come out, reducing its volume in the bottle, thus expanding the volume of the air inside the bottle. In a non-fizzy beverage, the expanding air reduces its pressure and "sucks the liquids back in", as equilibrium is established. No liquid flows out of the hole.
In a fizzy beverage, the expansion of the air volume makes it so that it isn't saturated anymore; the gas solved in the liquid can move out of the solution; the air in the bottle thus is both expanding and gaining particles. As long as these two factors balance each other, the air can expand without dropping in pressure, allowing the liquid to flow out. When the separation of the gas from the solution can't supply a number of molecules large enough to keep the pressure constant, we would fall back into the "non-fizzy beverage" case, stopping the flow.