Choose the correct statements about disinfection and softening of water.
A. The number of microorganisms destroyed in unit time is proportional to the number of microorganism remaining under ideal conditions -Chick's law.
B. Common disinfecting chemicals are $$Cl_2, NaOCl, and Ca(OCl)_2.$$​
C. When chlorine is added to water, mixture of HOCl and HCl is formed.
D. During chlorination, ammonia reacts with HOCl and forms chloramines.
E. Chlorine, exists in water in combination with organic nitrogen compounds, is referred to as combined non-available chlorine.
Choose the most appropriate answer from the options given below:

Correct option is C

Introduction

Water disinfection destroys pathogenic microorganisms, most commonly using chlorine, whose reactions in water produce various forms of available and combined chlorine.

Chick's law describes the kinetics of this microbial destruction.

Information Booster

The correct statements are A, B, C and D only.
A. Chick's law. Correct. Chick's law states that under ideal conditions the rate of destruction of microorganisms is proportional to the number of organisms remaining — a first-order kinetic relationship, expressed as dN/dt = −kN, meaning the die-off follows an exponential decay.
B. Common disinfecting chemicals. Correct. Standard water disinfectants include chlorine (Cl₂), ozone (O₃), and chlorine dioxide (ClO₂), among others such as chloramines and UV; the listed set represents the widely used chemical disinfectants.
C. Chlorine addition forms HOCl and HCl. Correct. When chlorine gas dissolves in water, it hydrolyses: Cl₂ + H₂O → HOCl + HCl, producing hypochlorous acid (HOCl, the primary disinfecting agent) and hydrochloric acid. This is the fundamental chlorination reaction.
D. Ammonia reacts with HOCl to form chloramines. Correct. In the presence of ammonia, hypochlorous acid reacts to form chloramines (NH₂Cl, NHCl₂, NCl₃) — the basis of chloramination, which provides a longer-lasting combined residual.

Additional knowledge

E. Chlorine combined with organic nitrogen is "combined non-available chlorine" — Incorrect as the discriminator.

While chlorine does combine with organic nitrogen compounds, the standard terminology treats combined available chlorine (chloramines) as still possessing disinfecting capacity; the statement's specific framing as "combined non-available chlorine" mislabels the accepted classification, so it is excluded from the correct set. Chlorine combined with ammonia/organic nitrogen as chloramines retains disinfecting ability and is termed combined available chlorine, not "non-available."