Hardness in water: Temporary hardness (which can be removed by boiling) is primarily due to the presence of which chemical species in solution?

Introduction / Context:
Water hardness affects scale formation, soap usage, and boiler performance. It is categorized as temporary (carbonate) hardness and permanent (non-carbonate) hardness. Engineers must identify which ions contribute to each type to choose an appropriate treatment method such as lime softening or ion exchange.

Given Data / Assumptions:

• Temporary hardness is removable by boiling or lime treatment.
• Hardness-forming cations are calcium and magnesium.
• Anions determine whether the hardness is temporary or permanent.

Concept / Approach:
Temporary hardness arises from bicarbonates (HCO3-) of calcium and magnesium. Heating converts bicarbonates to carbonate precipitates, releasing carbon dioxide and depositing CaCO3/Mg(OH)2, thus removing hardness—hence “temporary.” In contrast, sulphates and chlorides of calcium/magnesium cause permanent hardness that does not boil out. Therefore, the defining species is the carbonate alkalinity (bicarbonate/carbonate system), not merely the presence of calcium or sulphate/chloride ions alone.

Step-by-Step Solution:
Identify the temporary hardness mechanism: decomposition of bicarbonate on boiling.Relate to ions: carbonate alkalinity paired with Ca/Mg causes temporary hardness.Select “carbonate” as the correct defining species.

Verification / Alternative check:
Standard tests express temporary hardness approximately as alkalinity to phenolphthalein and methyl orange endpoints; pilot boiling confirms scale precipitation tied to carbonate chemistry.

Why Other Options Are Wrong:
Calcium: A cation present in both temporary and permanent hardness; the anion determines type.Chloride / sulphate: Produce permanent hardness with Ca/Mg and do not boil out.

Common Pitfalls:

• Equating “calcium” generically with temporary hardness.
• Overlooking that bicarbonate alkalinity is the key differentiator.

Final Answer:
carbonate