Why is bleaching powder used for disinfecting drinking water?

Question

Examoogle · Accepted Answer

The correct answer is Option D: It releases chlorine, which kills germs.. Bleaching powder (chemically known as calcium oxychloride, $CaOCl_2$) is widely used for disinfecting drinking water because it releases free chlorine when dissolved in water. This active chlorine acts as a powerful oxidizing agent that effectively destroys disease-causing pathogenic microorganisms such as bacteria and viruses, making the water safe for consumption.

Step-by-Step Solution & Concepts:
Bleaching powder (chemically known as calcium oxychloride, $CaOCl_2$) is widely used for disinfecting drinking water because it releases free chlorine when dissolved in water. This active chlorine acts as a powerful oxidizing agent that effectively destroys disease-causing pathogenic microorganisms such as bacteria and viruses, making the water safe for consumption.

Working Principle: When bleaching powder is added to water, it undergoes hydrolysis to release hypochlorous acid ($HOCl$) and calcium hydroxide. Hypochlorous acid is uncharged and penetrates the cell walls of pathogens easily, oxidizing their vital cellular enzymes and proteins, thereby killing them. The chemical reaction is represented as: $CaOCl_2 + H_2O \rightarrow Ca(OH)_2 + Cl_2$ (followed by $Cl_2 + H_2O \rightarrow HCl + HOCl$).

Key Formulas: $CaOCl_2 + H_2O \rightarrow Ca(OH)_2 + Cl_2$ — Hydrolysis of bleaching powder; $Cl_2 + H_2O \rightarrow HCl + HOCl$ — Formation of active disinfecting hypochlorous acid

Key Points: Bleaching powder is chemically known as Calcium oxychloride ($CaOCl_2$).; It acts as a disinfectant because it releases active chlorine upon reaction with water.; Chlorine gas and hypochlorous acid destroy bacterial cell walls and oxidize their metabolic enzymes.; It is also used in the textile and paper industries for bleaching cotton, linen, and wood pulp.

Advantages: Highly effective against a wide range of water-borne pathogens (bacteria, viruses, protozoa).; Inexpensive and easy to store, transport, and apply compared to compressed chlorine gas.

Disadvantages: Over-dosing can impart an unpleasant taste and odor to drinking water.; Can react with organic impurities to form undesirable disinfection byproducts (DBPs) like trihalomethanes.

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Option A: It reduces the hardness of water.

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Option B: It neutralises the water.

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Option C: It provides an acidic medium.

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