Dear student You get carbonated or fizzy water. The process requires that the cold/cool water be saturated with CO2. (when the CO2 returns to a gas, it is endothermic…requires input heat). Then when capped off while it forms more carbonic acid, H2CO3. H2O itself is also very stable - when there's no water around. When there is water around, it can decompose to H+ and OH-. Regards