How does chlorine act in water with a pH of 7?

Chlorine behaves in a specific way in water, particularly at a neutral pH of around 7. At this pH level, chlorine exists mainly as hypochlorous acid (HOCl), with a smaller proportion present as hypochlorite ions (OCl⁻). The equilibrium between these two forms is influenced by pH: at lower pH levels, more chlorine is in the form of hypochlorous acid, which is a stronger disinfectant than hypochlorite.

Hypochlorous acid is effective as a sanitizer and has a higher oxidation potential compared to hypochlorite ions, making it the primary disinfection agent in water treatment processes. Therefore, at a neutral pH, the presence of hypochlorous acid allows for efficient disinfection.

Other options present misunderstandings about chlorine behavior in water. Chlorine is not fully ineffective at neutral pH, nor does it exclusively form hydrochloric acid or convert entirely to chlorine gas under these conditions. Instead, hypochlorous acid is the dominant species, which is key to its effectiveness in water treatment.