Which one of the following gases is used for the purification of drinking water?

Chlorine is the gas used for disinfecting drinking water. It kills bacteria, viruses and other pathogens and leaves a residual effect in the distribution network, which is why it remains the most common method in municipal supplies. Chlorination of water began in the early twentieth century, and BIS standard IS 10500 still prescribes permissible residual chlorine limits.

• Sulphur dioxide (A): used as a bleaching agent and food preservative, and sometimes to dechlorinate water after treatment, but not as a primary purifier.
• Fluorine (C): extremely reactive and toxic as a gas, so it is never added to drinking water. Fluoride compounds are added in some places for dental health, which is a different thing.
• Carbon dioxide (D): used for recarbonation to correct pH after softening, and in beverages, not to kill microbes.

Core concept: the difference between disinfection, pH correction and dechlorination. Mains connection: GS-II and GS-III on public health, water-borne disease control and the Jal Jeevan Mission.

Trap: choosing sulphur dioxide because it is a familiar bleaching and preserving gas, or choosing fluorine because of the fluoridation debate.