Water hardness fundamentals: Temporary hardness in natural waters is primarily caused by the presence of which dissolved salts?

Introduction / Context:
Understanding the chemical origin of hardness helps select the correct treatment method. 'Temporary' hardness refers to species that decompose upon boiling or alkali addition, whereas 'permanent' hardness persists without chemical softening.

Given Data / Assumptions:

• Common cations: Ca^2+, Mg^2+.
• Common anions: HCO3^−, SO4^2−, Cl^−, CO3^2−.
• Definition: temporary hardness is removable by boiling (decomposition of bicarbonates).

Concept / Approach:
Bicarbonates of calcium and magnesium decompose upon heating to form insoluble carbonates and release CO2, eliminating hardness. Sulfates and chlorides remain dissolved and therefore contribute to permanent hardness. Sodium and potassium carbonates do not create hardness because Na^+ and K^+ are monovalent and do not form scale in the same way.

Step-by-Step Solution:
Temporary hardness salts: Ca(HCO3)2, Mg(HCO3)2.Upon boiling: Ca(HCO3)2 → CaCO3(s) + CO2 + H2O; Mg(HCO3)2 → MgCO3(s)/Mg(OH)2(s) + CO2 + H2O.Sulfates/chlorides do not decompose on boiling → permanent hardness.Therefore choose 'Bicarbonates of calcium and magnesium'.

Verification / Alternative check:
Classical titration of hardness (EDTA method) shows reductions after boiling due to precipitation of carbonate species only when bicarbonate hardness is present.

Why Other Options Are Wrong:
Chlorides/sulfates: do not cause temporary hardness; they contribute to permanent hardness.

Na/K carbonates: monovalent cations; do not impart hardness in the standard definition.

Common Pitfalls:

• Assuming all carbonate species are 'temporary'; the key is bicarbonate decomposition.
• Confusing alkalinity with hardness; they are related but distinct measures.

Final Answer:
Bicarbonates of calcium and magnesium