Question 1

Water softening agents – Calgon: Calgon (sodium hexametaphosphate or related polyphosphate) is primarily used for the removal/control of which type of water hardness?

Accepted Answer

Introduction / Context: Calgon is a trade name historically associated with sodium hexametaphosphate and related polyphosphates used as water conditioners. They act primarily by sequestering calcium and magnesium ions, preventing scale and 'permanent' hardness effects. Given Data / Assumptions: Hardness ions: Ca^2+, Mg^2+. Permanent hardness: typically sulfates/chlorides that do not decompose upon boiling. Application: domestic and industrial water conditioning. Concept / Approach: Polyphosphates complex Ca^2+/Mg^2+ to form soluble complexes, reducing precipitation of carbonates/sulfates and mitigating hardness effects without necessarily removing ions by precipitation. This is particularly effective for permanent hardness species. Step-by-Step Solution: Recognize Calgon as a sequestrant (complexing agent) for Ca^2+, Mg^2+.Identify target: permanent hardness (sulfates/chlorides) not removed by boiling.Therefore, Calgon is used against permanent hardness, not to remove Na2CO3 or K2CO3.Select option describing permanent hardness control. Verification / Alternative check: Product literature and water-conditioning texts confirm polyphosphates inhibit scale by binding hard ions, supporting heat-exchanger protection and detergent performance. Why Other Options Are Wrong: Removing sodium/potassium carbonate is not Calgon’s role; these salts are already soluble and do not create hardness. 'None of these' is incorrect because combating permanent hardness is the conventional use. Common Pitfalls: Confusing sequestration (keeping ions soluble) with true softening by precipitation/exchange. Ignoring polyphosphate reversion at high temperature/pH, which can reduce efficacy. Final Answer: Permanent hardness of water (sequestering Ca^2+ and Mg^2+)

Question 2

Drinking water disinfection – modern physical method: Which physical method is widely used as a germicidal treatment step in modern potable water systems?

Accepted Answer

Introduction / Context: Safe drinking water requires pathogen inactivation. While chemical disinfectants are common, a modern physical method that leaves no chemical residual and is effective against many microbes is ultraviolet (UV) irradiation, typically at germicidal wavelengths around 254 nm. Given Data / Assumptions: Continuous flow UV reactors sized for target UV dose (mJ/cm^2). Pre-treatment to reduce turbidity for adequate UV transmittance. Compliance with local regulatory standards for disinfection. Concept / Approach: UV radiation inactivates microorganisms by damaging nucleic acids, preventing replication. It is a physical method, contrasting with chlorination or bleaching powder (chemical) and permanganate (oxidant). UV is often paired with a secondary disinfectant residual for distribution safety. Step-by-Step Solution: Categorize options: chemical vs physical.Identify UV radiation as the sole physical germicidal option listed.Select UV as the correct answer.Note: utilities may use UV plus chloramination for residual. Verification / Alternative check: Utility practice and standards acknowledge UV systems for primary disinfection, especially effective against protozoa such as Cryptosporidium, which are chlorine-resistant. Why Other Options Are Wrong: Chlorination, bleaching powder, and permanganate are chemical oxidants/disinfectants, not physical methods. Common Pitfalls: Assuming UV provides a residual; a downstream chemical residual is usually still required. Ignoring UV transmittance limitations in colored/turbid waters. Final Answer: Ultraviolet (UV) radiation

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