Water quality – typical constituents causing hardness Which of the following dissolved species are responsible for the hardness of water commonly encountered in water-supply engineering?

Introduction / Context:
Hardness of water is primarily due to dissolved polyvalent cations, especially calcium and magnesium. Hardness affects soap consumption, scaling in boilers and pipes, and behavior in softening units. Recognizing which ions contribute allows engineers to select suitable treatment processes.

Given Data / Assumptions:

• Dissolved inorganic salts in natural waters.
• Hardness measured as mg/L as CaCO3 equivalent.
• Both temporary (carbonate) and permanent (non-carbonate) hardness considered.

Concept / Approach:
Temporary (carbonate) hardness arises mainly from bicarbonates of calcium and magnesium. Permanent hardness arises from sulphates, chlorides, and nitrates of calcium and magnesium. Therefore, calcium species and magnesium bicarbonate/sulphate are direct contributors to hardness.

Step-by-Step Solution:
Identify hardness-causing cations: Ca2+ and Mg2+ dominate.Link to anions present: HCO3- (temporary hardness), SO4^2- (permanent hardness), among others.All listed species either directly contain calcium/magnesium or are typical forms causing hardness.

Verification / Alternative check:
Standard water-chemistry texts define total hardness = calcium hardness + magnesium hardness, confirming the listed species contribute.

Why Other Options Are Wrong:

• Each of (a), (b), and (c) contributes; hence the comprehensive answer is 'All the above'.
• 'None of these' contradicts established chemistry.

Common Pitfalls:
Confusing alkalinity (capacity to neutralize acid) with hardness; although bicarbonates contribute to both concepts, they are not identical measures.

Final Answer:
All the above