11.4.2—Explain

When chlorine dissolves in water it establishes an equilibrium that produces hypochlorous acid: Cl₂ + H₂O ⇌ HCl + HOCl. HOCl can partially dissociate to H⁺ and OCl⁻.

HOCl and OCl⁻ are the active chlorine species that damage microorganisms. Their relative amounts depend on pH, so purification is a chemical equilibrium and a treatment decision, not simply “chlorine kills bacteria”.

A controlled chlorine dose leaves a measurable residual disinfectant after contact time. Lower pH shifts more active chlorine toward HOCl, but operational safety and local standards still constrain the choice.

Do not treat chloride ions as the disinfectant, or assume any chlorine dose is safe. Distinguish formation of active species from the public-health application.