The hardness of water is caused mainly by dissolved salts of

A $Na X^{+}$ and $K X^{+}$
B $Ca X^{2 +}$ and $Mg X^{2 +}$
C $NH X_{4} X^{+}$ and $H X_{3} O X^{+}$
D $Al X^{3 +}$ and $Zn X^{2 +}$

Answer: B

Hard water contains calcium and magnesium ions, which precipitate ordinary soaps as insoluble calcium and magnesium salts of fatty acids. Sodium and potassium soaps are soluble, so

Na X^{+}

and

K X^{+}

do not cause hardness.

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The hardness of water is caused mainly by dissolved salts of

Answer: B

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