Hardness of water – principal causative salts Hardness in potable water arises primarily from dissolved salts of calcium and magnesium. Which set below correctly reflects common hardness-causing species?

Introduction / Context:
Hardness is a key design parameter for softening plants and industrial water systems. It is typically expressed as mg/L as CaCO3 and arises from calcium and magnesium salts in different anionic forms.

Given Data / Assumptions:

• Natural groundwater or surface water sources.
• Both temporary (carbonate) and permanent (non-carbonate) forms present.
• No exotic heavy-metal contributions considered.

Concept / Approach:
Temporary hardness is mainly due to bicarbonates of calcium (and magnesium), removable by boiling or lime softening. Permanent hardness is caused by sulphates, chlorides, and nitrates of calcium/magnesium and needs chemical precipitation or ion exchange for removal.

Step-by-Step Solution:
Identify the cations: Ca2+ and Mg2+.Match forms: Ca(HCO3)2 → temporary hardness; CaSO4 and MgSO4 → permanent hardness.Thus all listed species cause hardness; select 'All the above'.

Verification / Alternative check:
Jar tests and standard methods demonstrate that bicarbonate alkalinity contributes to temporary hardness, whereas sulphates persist unless treated.

Why Other Options Are Wrong:

• Single-species options (a–c) are incomplete views.
• 'None of these' contradicts fundamental water chemistry.

Common Pitfalls:
Confusing hardness with alkalinity or TDS; ignoring magnesium contributions; designing softening only for calcium and overlooking magnesium load.

Final Answer:
All the above