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Difficulty: MediumPurification and Treatment of Water for Supply

During the municipal purification and treatment of river water for public supply, several chemical reagents are added at different stages of the process. Which of the following chemicals is correctly paired with its primary functional role in this water treatment scheme?

  1. Potash alum — Coagulating fine colloidal particles to form settleable flocsAnswer
  2. B
    Chlorine — Removing permanent hardness caused by dissolved calcium tetraoxosulfate(VI)
  3. C
    Slaked lime — Sterilizing the water by destroying pathogenic microorganisms
  4. D
    Activated carbon — Precipitating dissolved iron and manganese ions out of solution

Answer

Potash alum — Coagulating fine colloidal particles to form settleable flocs
Potash alum (aluminum potassium sulfate) serves as a coagulating agent. In raw river water, fine suspended clay particles carry negative electrical charges that keep them suspended. The addition of alum neutralizes these charges, causing the tiny particles to clump together into larger, heavier flocs that readily settle to the bottom during the sedimentation phase.

Step-by-Step Solution

1
Identify the primary purpose of each reagent in municipal water purification.
Potash alum KAl(SO4)2⋅12H2O\text{KAl(SO}_4\text{)}_2\cdot 12\text{H}_2\text{O} supplies Al3+\text{Al}^{3+} ions that neutralize negative charges on suspended colloidal clay particles.
This charge neutralization enables microscopic particles to aggregate (coagulate) into visible flocs.
2
Distinguish between physical particle coagulation, chemical softening, and disinfection.
Coagulation removes turbidity via alum, filtration removes flocs using sand beds, and chlorination destroys disease-causing micro-organisms.
Each chemical step serves a unique, non-interchangeable objective in town water supply treatment.

Key Concept

Chemical Reagent Functional Roles in Municipal Water Treatment
Estimated Time:1m 0s
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Topics

Purification and Treatment of Water for Supply

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