Why does shaking a bottle of soda/pop and then opening it cause the soda/ pop to come out the way it does? If the CO2 is added to the liquid under pressure, why does shaking it upset the equilibrium in the bottle?
Soda is a solution of a liquid component and a gaseous component, in this case CO2. The CO2 is dissolved in the liquid. At rest they are at equilibrium. There is a bit of space at the top of the bottle, and when you shake it, that space becomes bubbles, or pockets of air, which provide nucleation sites for the CO2 to escape from its dissolved state. The CO2 is now a gas, and because gas is compressible it exerts pressure on the bottle, hence the “explosion” when you open it.
In a sealed soda bottle you have CO2 dissolved in the liquid, then a big “bubble” of CO2 sitting on top of the liquid. If you open the bottle carefully, the big “bubble” sitting on top escapes straight out the neck of the bottle without disturbing the liquid beneath it. However, when you shake it up that great big bubble becomes a bunch of smaller bubbles suspended in the soda. Then when you open it, all those little bubbles suddenly become big bubbles as the pressure drops. The sudden increase in volume as the gas bubbles expand forces the gas-soda mixture out of the bottle.
This is partially accurate (the part about nucleation sites).
When you have a sealed bottle of carbonated soda at equilibrium (e.g. after having been sealed for at least several hours), shaking it will not cause any more CO2 to come out of solution. That is, shaking it does not alter the equilibrium: that is determined by the concentration of dissolved CO2 and the pressure of the gaseous CO2 above the soda.
If you want to get the CO2 out of solution, you need to relieve the pressure; you do this by opening the cap. Once the pressure is relieved, the dissolved CO2 is no longer in equilibrium: it wants to come out of solution, but it needs free surfaces (boundaries between liquid and gas) or other nucleation sites to do so.
If you haven’t shaken the soda, then there’s really only one major free surface, and that is the top surface of the soda. CO2 will come out of solution to air there, but not very quickly; that top layer of soda gets depleted, and then you’re relying on diffusion of dissolved CO2 from the rest of the bottle to transport dissolved CO2 up there. If you quickly recap your soda, CO2 will continue to come out of solution until the bottle is pressurized again, at which point you will have re-established equilibrium. OTOH, if you leave your fresh soda uncapped for a few hours, all of the CO2 will eventually come out of solution.
Note that once you open a bottle of soda, a few bubbles spontaneously form on the interior surface of the bottle. These are nucleation sites - little discontinuities in the bottle surface that encourage dissolution of gases when equilibrium is disturbed - and a bubble of CO2 will grow there until buoyant forces separate it from the surface, at which point it rises, and another bubble immediately begins to grow on that site. The famous Mentos+Coke trick works because the surface of Mentos candy is loaded with nucleation sites, encouraging extremely rapid dissolution of CO2 from the soda.
If you shake your soda before opening, as DCnDC notes, the big bubble of CO2 at the top of the bottle gets split into thousands of very small bubbles, creating a LOT of free surface for CO2 to come out of solution. However, none will actually come out of solution until you open the cap and relieve the pressure. Once you uncap it, you get rapid dissolution of CO2 at the surface of all those little bubbles.
Machine Elf - I may be mistaken but I think the Mentos reaction is more complex than just having nucleation sites. That’s why it works in Diet (with aspartame), to create a chemical reaction that facilitates the escape of CO2, but not with regular.
No, it works with regular soda as well, and as far as I know there is no chemical reaction taking place. However, here’s an article about a study which found that aspartame increased the effect by lowering surface tension.
The guy (or one of them, at least) who popularized the Mentos thing on Youtube says that the main reason he used diet soda is that it’s a lot less sticky if you get covered with the stuff.
So your’e right, there’s a bit more to it. Nucleation sites are the big factor, but reduced surface tension (due to Aspartame), surfactants on the surface of the candies, and the rapid sink rate of whole/intact Mentos candies all help things happen even faster.
And the size of the mints allows you to drop in several at the same time.
What causes a drink that’s already open to foam up if it gets set down too hard? For example, someone sets their drink on the table, but bumps it a little too hard.
The same kind of thing. If you slosh the drink around, air gets mixed into the liquid, providing extra nucleation sites for the CO[sub]2[/sub].
Why soda foam less if you’ve already poured a shot of alcohol into the glass? I’m guessing that it lowers the surface tension of the bubbles to the point where they can’t sustain very long.