Disinfecting water with chlorine gas. Chlorine is produced in the flask at left, by the reaction of hydrochloric acid with potassium permanganate. It bubbles into water that also contains universal indicator, making it green. The chlorine instantly bleaches the indicator colourless, and the green gas collects above the water. When more indicator is added, it turns red, indicating the presence of acid. The acids are hydrochloric and hypochlorous acid, which is reponsible for the disinfectant properties.