Question

With which of the following anions, $Mg^{2+}$ and $Ca^{2+}$ ions form salts responsible for permanent hardness of water?

• A. $Cl^-$, $SO_4^{2-}$
• B. $Cl^-$, $NO_2^-$
• C. $HCO_3^-$, $Cl^-$
• D. $CO_3^{2-}$, $HCO_3^-$

Hint:

Remember: "Bicarbonates are Boilable" (Temporary). "Sulfates and Chlorides are Constant" (Permanent). This mnemonic helps distinguish between the two types during exams.

Answer 1

The Correct Option is A

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].