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Difficulty: MediumChemistry in Daily Life and Industrial Applications

In industrial boiler operations and domestic water treatment, the presence of dissolved calcium and magnesium salts leads to boiler scale formation and reduced detergent efficiency. Which of the following chemical compounds is commercially known as 'Calgon' and softens hard water by forming soluble complex anions with calcium and magnesium ions?

  1. A
    Sodium carbonate decahydrate
  2. Sodium hexametaphosphateAnswer
  3. C
    Potassium aluminium sulfate
  4. D
    Calcium hypochlorite

Answer

Sodium hexametaphosphate is commercially known as Calgon and removes water hardness by sequestering calcium and magnesium ions into soluble complex anions.
Sodium hexametaphosphate (Na6P6O18Na_6P_6O_{18}) is commercially designated as Calgon. In aqueous solution, it ionizes to release [Na4P6O18]2−[Na_4P_6O_{18}]^{2-} complex anions, which bind Ca2+Ca^{2+} and Mg2+Mg^{2+} ions to form highly soluble complex salts such as [Na2CaP6O18]2−[Na_2CaP_6O_{18}]^{2-} and [Na2MgP6O18]2−[Na_2MgP_6O_{18}]^{2-}, preventing scale deposition in industrial boilers and improving lathering in cleaning applications.

Step-by-Step Solution

1
Identify the trade name and chemical identity
The commercial trade name 'Calgon' stands for 'calcium gone', which chemically corresponds to sodium hexametaphosphate, (NaPO3)6(NaPO_3)_6 or Na6P6O18Na_6P_6O_{18}.
Matching common industrial trade names to precise chemical nomenclature is a standard requirement in applied chemistry.
2
Analyze the chemical mechanism of water softening
When sodium hexametaphosphate is added to hard water, it dissociates as: Na6P6O18→2Na++[Na4P6O18]2−Na_6P_6O_{18} \rightarrow 2Na^+ + [Na_4P_6O_{18}]^{2-}. The complex anion exchanges sodium for calcium or magnesium: [Na4P6O18]2−+Ca2+→[Na2CaP6O18]2−+2Na+[Na_4P_6O_{18}]^{2-} + Ca^{2+} \rightarrow [Na_2CaP_6O_{18}]^{2-} + 2Na^+.
The sequestered calcium ions remain locked inside the soluble complex anion, preventing them from precipitating out as scale or forming insoluble scum with soaps.
3
Differentiate from alternative water treatment chemicals
Washing soda causes precipitation, alum causes coagulation of suspended particles, and bleaching powder is a disinfectant.
Distinguishing between precipitation, coagulation, disinfection, and complex ion sequestration confirms the unique role of Calgon.

Key Concept

Calgon Process and Complex Ion Formation in Water Softening
Estimated Time:1m 0s
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Topics

Chemistry in Daily Life and Industrial Applications

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