Hypochlorous acid from chlorine hydrolysis disinfects water

Steps:

• Chlorine gas (Cl₂) reacts with water: Cl₂ + H₂O ⇌ HOCl + HCl.
• HOCl forms and acts as the primary oxidant against bacteria.
• HOCl penetrates cell walls and disrupts metabolic processes.
• At neutral pH, HOCl is the dominant active species over its dissociated form.

Why D is correct:

• ClO represents hypochlorous acid (HOCl), a potent oxidant that kills bacteria by denaturing proteins, per oxidation-reduction chemistry.

Why the others are wrong:

• A. ClO⁻: Hypochlorite ion is less reactive and effective than undissociated HOCl for disinfection.
• B. Cl⁻: Chloride ion is a stable byproduct with no oxidizing or bactericidal ability.
• C. HCl: Hydrochloric acid contributes H⁺ ions but lacks the oxidative power to target bacteria.

Final answer: D