I would say the water is the solvent and carbon dioxide is the solute. Carbon dioxide is usually introduced to water under pressure and then sealed. Once the cap is removed, the carbon dioxide starts to escape since it is then under low pressure. Sometimes, natural groundwater has dissolved carbon dioxide in it but most of our soft drinks have it artificially introduced. Water plus carbon dioxide also form carbonic acid and this can give the tingly sensation on the tongue when drinking soft drinks.
