Correct Answer: B. Hypochlorous acid
Bleaching powder is a mixture of calcium hypochlorite, calcium chloride, and calcium hydroxide. When dissolved in water, it undergoes hydrolysis to release hypochlorous acid (HOCl), which is the primary active disinfectant. Hypochlorous acid is a weak acid that exists in equilibrium with its dissociated form (hypochlorite ion, OCl⁻). The undissociated HOCl molecule is the most potent antimicrobial agent because it can penetrate bacterial cell membranes more readily than the charged hypochlorite ion. HOCl denatures proteins, disrupts cell membranes, and damages nucleic acids through oxidative mechanisms. The disinfecting efficacy of bleaching powder is directly proportional to the concentration of available HOCl. This is why bleaching powder's disinfecting power decreases with storage, dilution, and exposure to light—all factors that reduce HOCl availability. In Indian public health practice, bleaching powder remains the most cost-effective disinfectant for water treatment and surface disinfection in resource-limited settings, and its mechanism of action is fundamentally dependent on HOCl generation.
Why the other options are wrong
A. Hydrochloric acid — HCl is not produced during bleaching powder hydrolysis and plays no role in its disinfecting action. While bleaching powder may contain trace HCl from manufacturing impurities, it is not the active disinfectant. This is a distractor that confuses the chlorine-containing acids; students may mistakenly think any acid with chlorine is responsible for disinfection. C. Hypochlorite ion — Although hypochlorite ion (OCl⁻) is produced from HOCl dissociation and does have antimicrobial properties, it is significantly less effective than the undissociated HOCl molecule. The charged hypochlorite ion cannot penetrate cell membranes as efficiently as the neutral HOCl. NBE may trap students who recognize that hypochlorite is present in bleaching powder but fail to distinguish between the ionized and unionized forms. D. Chloride ion — Chloride ion (Cl⁻) is a product of bleaching powder hydrolysis but is biologically inert and has no disinfecting properties. It is merely a spectator ion in the reaction. Students may select this if they confuse the presence of chlorine in the compound with antimicrobial activity, not realizing that only the oxidized chlorine species (HOCl and OCl⁻) are bactericidal.
High-Yield Facts
- Hypochlorous acid (HOCl) is the predominant disinfectant in bleaching powder, not the hypochlorite ion or chloride.
- Unionized HOCl penetrates bacterial cell membranes more effectively than the charged OCl⁻ ion, making it the most potent antimicrobial form.
- Bleaching powder disinfecting power decreases with storage, dilution, and light exposure due to loss of available HOCl.
- pH affects HOCl availability: at lower pH (acidic conditions), more HOCl is present; at higher pH, more OCl⁻ is present—HOCl is more effective.
- Bleaching powder is the WHO-recommended and most cost-effective disinfectant for water treatment in low-resource Indian settings.
- HOCl kills bacteria through oxidative damage to proteins, cell membranes, and nucleic acids.
Mnemonics
HOCl > OCl⁻ Rule Hypochlorous acid is Hotter (more active) than hypochlorite ion. The unionized form always penetrates membranes better than the charged form—remember this for all weak acids in disinfection.
NBE Trap
NBE pairs bleaching powder with hypochlorite ion to trap students who recognize that hypochlorite is present in the compound but fail to distinguish between the ionized (OCl⁻) and unionized (HOCl) forms—the latter is far more bactericidal.
Clinical Pearl
In Indian field settings, bleaching powder solution (0.5% available chlorine) is used for surface disinfection in hospitals and water purification in villages. Its effectiveness drops rapidly after opening because HOCl volatilizes and decomposes—this is why fresh bleaching powder must be used and stored in cool, dark containers.
_Reference: Park's Textbook of Preventive and Social Medicine, Ch. 5 (Water and Sanitation); Robbins Ch. 8 (Infectious Diseases)_