Most of us hear carbon and think chunks of black, burned material, not something found in Sprite or Pepsi. The carbon in carbonated water is carbon dioxide gas, and it doesn’t stay around the water forever. Carbon dioxide mixes with water only when the liquid is kept under pressure. When you pop open a bottle or can, that pressure is released, and the gas starts coming out of the water because normal atmospheric pressure isn’t strong enough to keep it in. The pressure in your stomach is insufficient, too, which is why pop makes you burp. Gradually all the carbon dioxide in carbonated water will release into the air. Result: flat pop. Carbonated water is known in chemistry circles as carbonic acid.
Source: Rudolph.S.Bottei, professor and assistant chair, Department of Chemistry and Biochemistry.