Water Quality – Temporary hardness of water Temporary hardness arises primarily due to the presence of which dissolved salts?

Introduction / Context:
Water hardness is caused by multivalent cations, mainly calcium and magnesium. It is categorized into temporary (carbonate) hardness and permanent (non-carbonate) hardness. Understanding the cause helps select the proper treatment method for domestic and industrial uses.

Given Data / Assumptions:

• Source water contains Ca^2+ and Mg^2+ with various anions.
• No unusual contaminants are assumed.
• Standard definitions of carbonate vs non-carbonate hardness apply.

Concept / Approach:

Temporary hardness is due to bicarbonates of calcium and magnesium (Ca(HCO3)2 and Mg(HCO3)2). It is called “temporary” because it can be removed by simple boiling, which converts bicarbonates to carbonates with precipitation of CaCO3/Mg(OH)2, or by lime-soda softening. Sulphates and chlorides contribute to permanent hardness and are not eliminated by boiling.

Step-by-Step Solution:

Verification / Alternative check:

Laboratory hardness split (carbonate vs non-carbonate) confirms that only the bicarbonate fraction diminishes on boiling, matching the definition of temporary hardness.

Why Other Options Are Wrong:

(b), (c), and (d) are salts associated with permanent hardness; (e) is not standard in hardness classification.

Common Pitfalls:

Assuming all hardness is removable by boiling or confusing alkalinity (bicarbonate) with total hardness; these are related but not identical measures.

Final Answer:

Bicarbonates of calcium (Ca) and magnesium (Mg)