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Carbonated water is $H_2 0$ with $H_2 CO_3$ in it. $H_2 CO_3$, carbonic acid, decomposes into water and $CO_2$, carbon dioxide. When the cap is closed on a bottle of carbonated water, there is a certain amount of $CO_2$ in the top of the bottle, where there is no liquid, and this circulates back into the liquid to become more carbonic acid. This level of $CO_2$ in the air pocket at the top stays more or less in a constant equilibrium though. When the top is removed and the $CO_2$ is allowed to escape, more carbonic acid has to break down than is formed to maintain equilibrium of $CO_2$ levels with the surface. I understand this whole process, but there is one thing I can't wrap my head around.

When more carbonic acid is breaking down to form $CO_2$, why does that cause bubbles everywhere in the liquid? How is it that when the level of $CO_2$ on the surface drops, it immediately causes carbonic acid to decompose everywhere in the liquid? Is it a chain reaction emanating down from the surface or is there some other means of communicating this to everything below the top layer?

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The carbon dioxide in a fizzy drink is mostly just dissolved in the water and not in the form of carbonic acid. The equilibrium constant for the reaction:

$$ CO_2 + H_2O \rightarrow H_2CO_3 $$

is $1.7 \times 10^{-3}$, so only about $0.2$% of the dissolved $CO_2$ forms carbonic acid. The other 99.8% is just a dissolved gas like any gas dissolved in water.

The solubility of most gases in water increases with pressure and this is true of carbon dioxide. So when you twist open the cap and the pressure is released the contents immediately become a supersaturated solution of carbon dioxide and it is thermodynamically favourable for the carbon dioxide to come out of solution.

The $CO_2$ doesn't all immediately bubble out of solution because there is an energy barrier for the formation of a bubble in the solution. I describe the physics behind this in my answer to Why does soda fizz when it meets ice?. If you look carefully at the bottle while you're opening it you'll find the bubbles tend to form on the bottle walls where tiny defects in the glass or plastic can act as nuclei.

If you're interested there is some related information in Why are coke bottles that much fizzier if you shake them?.

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  • $\begingroup$ Thanks. It was actually looking at bubbles forming at the little defects in a glass that this question came to me. $\endgroup$ – Ulthran Sep 22 '16 at 16:01
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when $CO_2$ is produced it can't dissolve back into the soda, and because of the surface tension it will form bubbles, same as water forms bubbles when it is in flying with the air. What triggers the process when you open the bottle is that the pressure decreases(soda cans have a lot of pressure before you open them) and because of Henry's law($H_d=b/pressure$) the amount of $CO_2$ that can be dissolved in the water drops, thus liberating $CO_2$ in the air. So the important thing here is pressure, not $CO_2$ concentration. For example, if you close the bottle again $CO_2$ increases the pressure of the bottle and it stops producing bubbles.

EXTRA: underwatter, pressure is so high that soda doesn't create bubbles, here is cool video about it:

https://www.youtube.com/watch?v=EJiUWBiM8HE

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