What gas makes soda bubbly?

Carbon dioxide is the gas that makes soda bubbly. Manufacturers dissolve CO₂ in the drink under pressure, allowing much more gas to remain in the liquid than would normally dissolve at atmospheric pressure.

When a bottle or can is opened, the pressure above the liquid drops. The soda can no longer hold as much dissolved gas, so carbon dioxide escapes through tiny nucleation sites—small scratches, particles, or imperfections—forming the familiar bubbles.

Some of the dissolved CO₂ reacts with water to form a small amount of carbonic acid, which contributes to the sharp, slightly tart taste of carbonated drinks. Carbon dioxide is not carbon monoxide: carbon monoxide is a poisonous gas and is not used to carbonate beverages. The same basic process gives sparkling water, champagne, and some beers their fizz.