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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