The difference between the amount of chlorine added to water and the amount of residual chlorine after a specified contact period is defined as:

Correct option is B

Explanation:
Chlorine demand is defined as the difference between the amount of chlorine added to water and the amount of residual chlorine remaining after a certain period of contact time.
Chlorine Demand=Chlorine Applied−Residual Chlorine
This value represents the amount of chlorine consumed by reactions with:
· Organic matter
· Inorganic substances (e.g., iron, manganese, hydrogen sulfide)
· Microorganisms in the water
Information Booster:
· Residual chlorine is the chlorine remaining in water after the demand is met.
· It ensures continued disinfection as water travels through pipes.
· Chlorine demand is important in water treatment processes to determine the correct chlorine dosage.
Additional Information:
· (a) Combined available chlorine: Chlorine that has combined with nitrogen compounds, such as chloramines.
· (c) Free available chlorine: Chlorine available in water in free form (not combined), mainly as HOCl and OCl⁻.
· (d) Total chlorine: Sum of free and combined chlorine present in water.