Which type of chlorine residual is produced by the reaction of chlorine with substances in water?

The correct choice is based on understanding the processes that occur when chlorine is added to water. When chlorine is introduced to water, it does not simply remain in its free form. Instead, it reacts with various organic and inorganic compounds present in the water, such as ammonia and other nitrogenous materials, leading to the formation of chloramines and other chlorine derivatives. This reaction produces what is known as combined chlorine residual.

Combined residual refers specifically to the portion of chlorine that has reacted with these substances and is no longer available to provide disinfection but is still present in the water. It is a critical factor to consider in water treatment because it can influence the efficiency of the disinfection process and the overall water quality. Understanding the balance between free and combined chlorine residual is vital for effective water treatment and maintaining safety standards.