When is free chlorine residual typically formed?

Free chlorine residual is typically formed when chlorine reacts with water to produce hypochlorous acid (HOCl) and hypochlorite ion (OCl-). This reaction occurs because chlorine is a strong oxidizing agent and when added to water, it disassociates to form these two species, which are responsible for the disinfection properties of chlorine.

Hypochlorous acid is the more effective antimicrobial agent among the two, while hypochlorite ion is also effective under certain pH conditions. The presence of these substances is what constitutes the free chlorine residual, which is crucial for ensuring the ongoing disinfection of water. Monitoring free chlorine residual is important in water treatment processes to verify that water maintains adequate disinfectant levels throughout distribution systems.

The other options describe scenarios that do not directly relate to the formation of free chlorine residual. Heating chlorine can change its state but does not facilitate the formation of hypochlorous acid and hypochlorite ion. Chlorine reacting with ammonia actually leads to the formation of chloramines, which are a different type of compound and do not contribute to the free chlorine residual. Finally, chlorine evaporation would simply remove chlorine from the water rather than creating any residual.