The gas (bubbles) in soft drinks is carbon dioxide (CO2).

The carbon dioxide is added to soft drinks at the end of the production process. The gas is added at high pressure through the drink, until the liquid is saturated with the gas. Carbon dioxide readily dissolves into water to form carbonic acid H2CO3.

When the pressure is released the acid will again decompose to carbon dioxide gas. This process will continue to a state of equilibrium. In a closed can or bottle the air (headspace) and the liquid are saturated with the gas. When opened the equilibrium is disturbed, as the carbon dioxide will disappear from the headspace. This results in more formation from carbon dioxide from the liquid, which is released as bubbles. When the bottle is closed the bubbles will disappear slowly when the state of equilibrium is reached again.

Shaking the can or bottle disturbs the equilibrium and causes a lot of gas to be liberated. This results in an overpressure in the headspace, and a fountain when the bottle is opened. In time the overpressure is reduced, as the carbon dioxide again dissolves into the liquid.

Carbon dioxide is also slowly dissolving in some spring water, especially in areas with a high percentage of chalk in the soil. The chalk, calcium carbonate, slowly dissolves into the water as carbonic acid. When the water surfaces the pressure is released and the gas is liberated.

Beer and sparkling wines also contain carbon dioxide, which is produced by the yeast during the fermentation process.