In disinfection, which of the following forms of chlorine is most effective in killing the pathogenic bacteria?
- ACl
- BOCl
- CNH2Cl
- DHOClCorrect
Solution
Concept
Hypochlorous acid (HOCl) is the strongest disinfectant among the free and combined chlorine forms.
Formula
Cl₂ + H₂O → HOCl + H⁺ + Cl⁻ ; HOCl ⇌ H⁺ + OCl⁻
Step-by-step solution
1. When chlorine is added to water, it forms HOCl. 2. HOCl partly breaks into OCl⁻, depending on pH. 3. HOCl is about 80-100 times more effective than OCl⁻. 4. Chloramines (NH₂Cl) are much weaker. 5. So HOCl is the most effective form. Answer is D.
Explanation
HOCl is a neutral molecule, so it easily enters bacterial cells and kills them. OCl⁻ and chloramines are much weaker disinfectants. That is why chlorination works best at lower pH, where more HOCl is present.
Why the other options are wrong
A: Cl (chloride ion or chlorine) is not the main killing agent. B: OCl⁻ is about 80-100 times less effective than HOCl. C: NH₂Cl (monochloramine) is a weak, slow-acting combined chlorine.
Shortcut
Order of strength: HOCl > OCl⁻ > chloramines.
Common mistake
Students choose OCl⁻ because it is a free chlorine form. HOCl is much stronger.
Correct answer: D) HOCl