Question

Which of the following ions are responsible for hardness of water?

• A. \(\text{Ag}^+,\text{Au}^{3+}\) ions
• B. \(\text{Hg}^{2+},\text{Pb}^{2+}\) ions
• C. \(\text{Ca}^{2+}\) and \(\text{Mg}^{2+}\) ions.
• D. \(\text{As}^{3+},\text{CN}^-\)

Hint:

Hardness comes from dissolved calcium and magnesium salts.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Hard water does not form lather easily with soap. The cause is dissolved bicarbonates, chlorides and sulphates of calcium and magnesium.

Step 2: Explanation:
Soap is sodium stearate. The \(\text{Ca}^{2+}\) and \(\text{Mg}^{2+}\) ions react with it to form insoluble scum, so soap is wasted before lather forms.
\[ 2\text{C}_{17}\text{H}_{35}\text{COONa} + \text{Ca}^{2+} \to (\text{C}_{17}\text{H}_{35}\text{COO})_2\text{Ca}\downarrow + 2\text{Na}^+ \]

Step 3: Other options:
\(\text{Ag}^+\), \(\text{Au}^{3+}\), \(\text{Hg}^{2+}\), \(\text{Pb}^{2+}\), \(\text{As}^{3+}\) and \(\text{CN}^-\) are not present in natural water as hardness agents. Some are toxic pollutants, but they do not cause hardness.

Final Answer:
Hardness is due to \(\text{Ca}^{2+}\) and \(\text{Mg}^{2+}\) ions, which is option (C). \[ \boxed{\text{Ca}^{2+}\ \text{and}\ \text{Mg}^{2+}} \]