Which statement about ammonia and chlorine is true?

• A. Ammonia does not interact with chlorine.
• B. Chlorine alone completely eliminates ammonia.
• C. Ammonia increases chlorine's disinfectant strength.
• D. Ammonia reacts with chlorine to form chloramines, which are weaker disinfectants but more persistent.

Answer: (D)

Ammonia and chlorine interact to form chloramines. When chlorine is added to water that contains ammonia, it reacts to produce chloramines (such as monochloramine and dichloramine). These chloramines are weaker disinfectants than free chlorine, but they persist longer in the water, providing a longer-lasting residual disinfectant in the distribution system. That’s why this statement is true: ammonia does react with chlorine to form chloramines, and those chloramines are weaker disinfectants but more persistent. The other options don’t fit because chlorine does not completely eliminate ammonia on its own, ammonia does interact with chlorine, and ammonia does not increase chlorine’s disinfection strength.