Permanent hardness in natural waters is primarily caused by which dissolved salts of calcium and magnesium?

Introduction / Context:Water hardness affects scaling in boilers, heat exchangers, and household appliances. Distinguishing temporary from permanent hardness guides the selection of treatment processes (e.g., lime softening versus ion exchange). Permanent hardness persists on boiling and is associated with specific salts.

Given Data / Assumptions:

• Hardness refers to divalent cations, mainly Ca2+ and Mg2+.
• Temporary hardness comes from bicarbonates that decompose on boiling.
• Permanent hardness remains after boiling due to non-bicarbonate anions.

Concept / Approach:Permanent hardness is due to chlorides and sulfates of calcium and magnesium. These salts do not decompose upon heating and therefore require chemical precipitation (soda ash) or ion exchange for removal. In contrast, bicarbonates (temporary hardness) release CO2 and precipitate as carbonates during boiling or lime treatment.

Step-by-Step Solution:

Verification / Alternative check:Analytical hardness calculations separate carbonate and non-carbonate hardness; the latter correlates with chloride and sulfate concentrations of Ca and Mg.

Why Other Options Are Wrong:

• Bicarbonates: Temporary hardness, removable by boiling.
• Sodium/potassium salts: Monovalent cations do not cause hardness.
• Phosphates of Na/K: Not a standard cause of hardness in potable waters.

Common Pitfalls:Equating TDS with hardness; high TDS water may have low hardness if dominated by sodium salts.

Final Answer:chlorides and sulfates of calcium and magnesium