Question:
With which of the following anions, $Mg^{2+}$ and $Ca^{2+}$ ions form salts responsible for permanent hardness of water?
With which of the following anions, $Mg^{2+}$ and $Ca^{2+}$ ions form salts responsible for permanent hardness of water?
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Remember: "Bicarbonates are Boilable" (Temporary). "Sulfates and Chlorides are Constant" (Permanent). This mnemonic helps distinguish between the two types during exams.
Updated On: Jul 1, 2026
- $Cl^-$, $SO_4^{2-}$
- $Cl^-$, $NO_2^-$
- $HCO_3^-$, $Cl^-$
- $CO_3^{2-}$, $HCO_3^-$
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The Correct Option is A
Solution and Explanation
Hardness in water is caused by the presence of dissolved multivalent metallic cations, primarily calcium and magnesium. Hardness is classified into two types: temporary and permanent.
1. Temporary Hardness:
This is caused by the bicarbonates ($HCO_3^-$) of calcium and magnesium. It is called "temporary" because it can be easily removed by boiling.
2. Permanent Hardness: This is caused by the presence of soluble
chlorides ($Cl^-$) and
sulfates ($SO_4^{2-}$) of calcium and magnesium[cite: 74]. These cannot be removed by boiling and require chemical treatment like ion exchange or the use of washing soda.
3. Specific Salts Involved: Typical salts responsible for permanent hardness include:
• Calcium Chloride ($CaCl_2$) [cite: 74]
• Magnesium Chloride ($MgCl_2$) [cite: 74]
• Calcium Sulfate ($CaSO_4$) [cite: 74]
• Magnesium Sulfate ($MgSO_4$) [cite: 74]
Therefore, the anions responsible for permanent hardness are $Cl^-$ and $SO_4^{2-}$[cite: 74].
2. Permanent Hardness: This is caused by the presence of soluble
chlorides ($Cl^-$) and
sulfates ($SO_4^{2-}$) of calcium and magnesium[cite: 74]. These cannot be removed by boiling and require chemical treatment like ion exchange or the use of washing soda.
3. Specific Salts Involved: Typical salts responsible for permanent hardness include:
• Calcium Chloride ($CaCl_2$) [cite: 74]
• Magnesium Chloride ($MgCl_2$) [cite: 74]
• Calcium Sulfate ($CaSO_4$) [cite: 74]
• Magnesium Sulfate ($MgSO_4$) [cite: 74]
Therefore, the anions responsible for permanent hardness are $Cl^-$ and $SO_4^{2-}$[cite: 74].
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