Step 1: Understanding the Concept:
Hardness in water is caused by the presence of dissolved salts of multivalent metal ions, primarily calcium (Ca$^{2+}$) and magnesium (Mg$^{2+}$). There are two types of hardness:
- Temporary hardness: Caused by bicarbonates of calcium and magnesium. It can be removed by boiling.
- Permanent hardness: Caused by chlorides and sulfates of calcium and magnesium. It cannot be removed by boiling.
The question asks to identify the salts responsible for permanent hardness.
Step 2: Detailed Explanation:
Based on the definition:
- We need to look for chlorides or sulfates of calcium (Ca) or magnesium (Mg).
- Salts of monovalent ions like sodium (Na$^+$) and potassium (K$^+$) do not cause hardness.
- Salts of other multivalent ions like aluminum (Al$^{3+}$) can contribute to hardness but Ca$^{2+}$ and Mg$^{2+}$ are the primary culprits.
Let's analyze the options:
- (A) NaCl, KCl: Both are salts of monovalent cations (Na$^+$, K$^+$). They do not cause hardness.
- (B) CaCl$_2$, KCl: CaCl$_2$ causes permanent hardness, but KCl does not.
- (C) AlCl$_3$, MgCl$_2$: MgCl$_2$ causes permanent hardness. AlCl$_3$ is not typically considered a primary cause of water hardness.
- (D) MgCl$_2$, CaCl$_2$: Both magnesium chloride (MgCl$_2$) and calcium chloride (CaCl$_2$) are chlorides of the primary hardness-causing ions. Both cause permanent hardness.
Step 3: Final Answer:
The pair of chlorides responsible for permanent hardness of water is MgCl$_2$ and CaCl$_2$. Therefore, option (D) is correct.