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For a sample of temporary hard water, which water-softening method does NOT produce any precipitate responsible for hardness?

1. Boiling of water
2. Addition of lime water
3. Addition of sodium hexametaphosphate
4. Addition of sodium carbonate

Correct answer: Addition of sodium hexametaphosphate

Solution

Sodium hexametaphosphate (calgon, [NaPO3]6) forms stable, soluble complexes with Ca²+ and Mg²+ ions without any precipitation. All other listed methods remove hardness by producing insoluble precipitates such as CaCO3 or Mg(OH)2.

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