Carbonic acid leaves the soda solution as CO2 (carbon dioxide). When a soda goes flat, all the CO2 leaves, however the H+ (acidic protons) are still left in the solution, leaving the solution acidic but flat. Solution Carbonic acid leaves the soda solution as CO2 (carbon dioxide). When a soda goes flat, all the CO2 leaves, however the H+ (acidic protons) are still left in the solution, leaving the solution acidic but flat..