# Why do soft drinks fizz?

I know it has something to do with carbon dioxide, but what exactly happens and causes the characteristic sound we hear (especially after we shake the bottle)? How do we manage to dissolve a non-polar substance ($\ce{CO2}$) in a polar solvent ($\ce{H2O}$)?

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– user15489
Jun 23, 2015 at 10:57
• Decrease of pressure causes less-solubility of gas. the gas try to come out from liquid medium. Nov 8, 2016 at 10:02
• $\ce{CO2}$ is quite polar. Sep 29, 2017 at 5:10

Because $\ce{CO_2}$ isn't exactly what's being dissolved. When it encounters water, it immediately encounters the following potential reaction:

$$\ce{CO2~(g) + H2O~(l) <=> H2CO3~(aq)}$$

Carbon dioxide is non polar, yes, but carbonic acid readily dissolves (and dissociates, but I didn't care to show that) in water.

The sound we hear is because when we open the bottle, the equilibrium above shifts to the left, creating more carbon dioxide that is being released. Why does the equilibrium shift? It has to do with the pressure in the bottle.

When the bottle is closed, it's under a certain set pressure. When it's closed at the same temperature and pressure for a long time, the reaction above reaches an equilibrium state, where the forward and backward versions of the reaction are happening at the same rate. However, when the bottle is opened, the pressure inside the bottle decreases, as the gas in the bottle suddenly has a lot more space to expand into. A decrease in pressure favors the side of an equilibrium that has more gaseous molecules, which in this case means the carbon dioxide. Ergo, when you open the bottle, the gas is formed, escapes from solution, and rapidly expands outward, making the infamous popping noise.

• It may be a little too much hand-holding, but this answer does rely a lot on the meaning of $\leftrightharpoons$ which may not be clear to non-chemists. Explicitly stating immediately after the equation that it’s a two-way street would make the discussion about equilibrium more clear to those unfamiliar with the symbol. Jun 22, 2015 at 19:02
• You explain why decreasing the pressure makes it fizz, but now I'm wondering why shaking the drink makes it fizz more. Jun 22, 2015 at 21:52
• I think the main effect of shaking isn't to make the gas come out faster, but to make more liquid be carried along with the gas.
– bdsl
Jun 23, 2015 at 7:42
• States of aggregation should not be subscripted, it is not wrong, but the recommendations (Sec. 2.1.) are different. Jun 23, 2015 at 10:53
• There is barely any carbonic acid in carbonated water, the equilibrium constant is just too low en.wikipedia.org/wiki/… Jul 5, 2016 at 22:43

Why do soft drinks fizz?

The key word for that is pressure

Manufactures close the bottles/cans by forcing carbon dioxide and water into your soda at high pressures around 40 psi.

When you crack open the can, you release the pressure and allow the gas bubbles to wiggle free from the liquid and rise to the surface. This requires energy because in order for the gas to break free from the liquid it has to overcome the force holding the liquid together.

Source: LIVESCIENCE