How will the pH water change if you add carbon dioxide?

Just stick the litmus paper in club soda (carbonated water). CO2 will change the pH of the water to roughly 5.7. This is only because SOME of the CO2 will react with the H+ atoms to form carbonic acid, lowering the solutions pH slightly. The pH scale represents the amount of H+ in a given substance. Likewise, the pOH scale represents the amount of OH- in a given substance (exact opposite of the pH scale).