Cause of hardness in water (expressed as CaCO3):\r Hardness of water is primarily caused by which dissolved constituents?

Introduction / Context:
Hardness is a key aesthetic and operational parameter in potable water treatment. It increases soap consumption, promotes scaling in boilers and heaters, and affects corrosion control strategies. Knowing its chemical origin is fundamental to selecting appropriate treatment such as lime-soda softening or ion exchange.

Given Data / Assumptions:

• Total hardness is reported as CaCO3 equivalent.
• Divalent cations calcium (Ca2+) and magnesium (Mg2+) are the principal contributors.
• Counter-ions commonly encountered are bicarbonate (temporary hardness) and sulphate/chloride (permanent hardness).

Concept / Approach:
Temporary hardness is mainly due to bicarbonates of Ca and Mg; permanent hardness arises from their chlorides and sulphates. Sodium and potassium salts do not precipitate soaps and thus do not cause classical hardness problems. Suspended solids (turbidity) are unrelated to chemical hardness.

Step-by-Step Solution:
Identify causative ions: Ca2+ and Mg2+ combined with HCO3–, SO4^2–, or Cl–.Relate to observed effects: scaling and soap precipitation.Select the option listing these salts explicitly.

Verification / Alternative check:
Soap titration tests and hardness calculations express results as CaCO3 equivalent precisely because Ca and Mg dominate conventional hardness behavior.

Why Other Options Are Wrong:

• Soap lather is an effect, not a cause.
• Sodium/potassium salts do not precipitate soaps and rarely form scale under normal conditions.
• Turbidity is a physical parameter, not chemical hardness.
• Silica/iron may cause other issues but are not principal hardness agents.

Common Pitfalls:

• Confusing alkalinity (HCO3–) with hardness; alkalinity can exist without hardness and vice versa.
• Assuming “softening” always means demineralization; many processes exchange Ca/Mg for Na+.

Final Answer:
Bicarbonates, sulphates, or chlorides of calcium and magnesium