The ions responsible for permanent hardness in water are sulphates of

Analysis

The question asks for the specific ions that cause permanent hardness in water, specifically in the form of sulphates.

Concept Explanation

Water hardness is classified into two types:

1. Temporary Hardness: Caused by the presence of dissolved bicarbonates of calcium or magnesium. This can be removed by boiling.
2. Permanent Hardness: Caused by the presence of dissolved chlorides and sulphates of calcium ($Ca^{2+}$) and magnesium ($Mg^{2+}$). This cannot be removed by boiling; it requires chemical treatment (like adding washing soda or using ion-exchange resins).

Step-by-Step Solution

• Step 1: Identify the definition of permanent hardness. It is the hardness that remains after boiling, caused by salts that do not precipitate upon heating.
• Step 2: Identify the chemical species involved. The primary culprits are the calcium ion ($Ca^{2+}$) and the magnesium ion ($Mg^{2+}$).
• Step 3: Match these ions with the anions mentioned in the question (sulphates). The compounds are Calcium Sulphate ($CaSO_4$) and Magnesium Sulphate ($MgSO_4$).
• Step 4: Evaluate the options. • Option A: Iron is not the primary cause of water hardness. • Option B: Contains $Ca^{2+}$ and $Mg^{2+}$, which are the correct ions. • Option C: Iron ions are not the standard cause of water hardness. • Option D: Sodium ($Na^+$) does not cause water hardness; in fact, sodium salts are often used to soften water.

Conclusion

The ions responsible for permanent hardness in water are the calcium and magnesium ions.

Exam Tip: Remember the mnemonic "CaMg" (pronounced like "Cam-Gee"). Whenever you see questions about water hardness, look for Calcium ($Ca$) and Magnesium ($Mg$). If the question asks for temporary hardness, look for bicarbonates; if it asks for permanent hardness, look for chlorides or sulphates.