What reacts with chlorine to create combined chlorine?

Combined chlorine is formed when chlorine reacts with ammonia or nitrogen-containing compounds in water. This reaction results in various chloramines, which include nitrogen trichloride, dichloramine, and monochloramine. These compounds are referred to as combined chlorine because they are formed from the reaction of free chlorine with nitrogenous compounds, significantly affecting the water quality and disinfection process.

The presence of chloramines can often be a concern in water treatment facilities because they are less effective as disinfectants than free chlorine and can contribute to taste and odor issues. Understanding combined chlorine is crucial for managing water treatment processes effectively, ensuring that water remains safe for consumption and uses.

The other options do not directly relate to the formation of combined chlorine:

• Sodium thiosulfate is a reducing agent used to neutralize chlorine, not to form it.

• Ammonium chloride is a source of ammonia that could form combined chlorine, but it does not directly create the chloramines.

• Calcium carbonate is a mineral that primarily affects the hardness and alkalinity of water rather than chlorine chemistry.

Thus, the various forms of chloramines generated, represented in the selected option, correctly indicate the species that result from the reaction of chlorine with ammonia or nitrogenous compounds in water.