Question

Which among the following is the active form of chlorine responsible for disinfection during chlorination?

• A. Hypochlorite ion
• B. Hydrogen chloride
• C. Hypochlorous acid
• D. Chloride ion

Hint:

Free chlorine kills mainly via uncharged hypochlorous acid (HOCl).

Answer 1

The Correct Option is C

Step 1: Trace what happens in water. Chlorine reacts with water to yield two species in equilibrium - undissociated hypochlorous acid $HOCl$ and its dissociated form, the hypochlorite ion $OCl^-$. The germicidal punch comes from $HOCl$.
Step 2: Quantify the difference. Hypochlorous acid is about 70 to 80 times stronger as a disinfectant than the hypochlorite ion, making it the active fraction of free residual chlorine.
Step 3: Explain mechanistically. Because $HOCl$ carries no charge and is small, it slips through the microbial membrane and oxidises vital cell components, whereas the charged $OCl^-$ is repelled.
Step 4: Knock out the rest. Hydrogen chloride and the chloride ion play no disinfecting role, and the hypochlorite ion is the weaker partner - leaving hypochlorous acid as the answer.
\[\boxed{\text{Hypochlorous acid}}\]