What effect will the addition of chlorine gas have on the pH of the water?

• A. Higher
• B. No change
• C. Increase
• D. Lower

Answer: (D)

When chlorine gas is added to water, it reacts with water to form hydrochloric acid and hypochlorous acid. The hydrochloric acid dissociates completely to release hydrogen ions, and hypochlorous acid also contributes hydrogen ions through its dissociation. The net result is an increase in acidity, which means the pH decreases. So the effect is a lower pH.