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Select the statement that is correct: Polyphosphates are used as water-softening agents because they

1. form soluble complexes with anions
2. cause cationic species to precipitate
3. bring about precipitation of anionic species
4. form soluble complexes with cations

Correct answer: form soluble complexes with cations

Solution

Polyphosphates soften water by forming soluble complexes (sequestering) with the Ca2+/Mg2+ cations, not by precipitating anionic species.

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