2.  The most popular pool disinfectant is the
element chlorine, in the form of a chemical
compound such as calcium hypochlorite (a
solid) or sodium hypochlorite (a liquid).
 When the compound is added to the
water, the chlorine reacts with the water to
form various chemicals, most notably
hypochlorous acid. Hypochlorous acid kills
bacteria and other pathogens that survive in
water.

3.  When chlorine is added to water, it reacts to
form a pH dependent equilibrium mixture of
chlorine, hypochlorous acid and hydrochloric
acid:
Cl2 + H2O → HOCl + HCl
 Depending on the pH, hypochlorous acid
partly dissociates to hydrogen and
hypochlorite ions:
HOCl → H + ClO