Question

Which of the following salts is the main cause of permanent hardness of water ?

• A. Magnesium sulphate
• B. Magnesium bicarbonate
• C. Magnesium carbonate
• D. Calcium Sulphate

Hint:

Remember:
- Carbonates and Bicarbonates of Calcium and Magnesium cause "Temporary Hardness" (removable by boiling).
- Sulfates and Chlorides of Calcium and Magnesium cause "Permanent Hardness" (not removable by boiling).

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
Water hardness is caused by the presence of dissolved multivalent metallic cations, primarily calcium (\(Ca^{2+}\)) and magnesium (\(Mg^{2+}\)).
Hardness is classified into temporary hardness (which can be removed by boiling) and permanent hardness (which cannot be removed by boiling).

Step 2: Detailed Explanation:
Temporary hardness is caused by the presence of dissolved bicarbonate and carbonate salts of calcium and magnesium (e.g., \(Ca(HCO_3)_2\), \(Mg(HCO_3)_2\), \(CaCO_3\), \(MgCO_3\)).
Boiling decomposes these soluble bicarbonates into insoluble carbonates, which precipitate out of the water.
Permanent hardness is caused by the presence of sulfate and chloride salts of calcium and magnesium (e.g., \(CaSO_4\), \(MgSO_4\), \(CaCl_2\), \(MgCl_2\)).
These salts do not precipitate upon heating, so boiling cannot remove them.
Among these, Calcium Sulphate (\(CaSO_4\)) is the most common salt causing permanent hardness in natural waters.
Therefore, while both Calcium Sulphate and Magnesium Sulphate cause permanent hardness, Calcium Sulphate is widely considered the primary salt responsible for permanent hardness in natural water systems.

Step 4: Final Answer:
Calcium Sulphate is the primary salt responsible for permanent hardness in water.