Temporary hardness of water – principal chemical cause Which dissolved species are primarily responsible for temporary hardness in water supplies?

Introduction:
Hardness in water is classified as temporary (carbonate hardness) and permanent (non-carbonate hardness). Understanding the chemistry is essential for selecting appropriate treatment methods such as boiling, lime softening, or ion exchange.

Given Data / Assumptions:

• Potable water chemistry in a typical municipal context.
• Hardness expressed as equivalent CaCO3 mg/L.
• Focus on carbonate system species at normal pH ranges.

Concept / Approach:

Temporary hardness arises due to bicarbonates (HCO3−) and carbonates (CO3^2−) associated with calcium and magnesium. When heated (boiling) or treated with lime, these species precipitate as carbonates (e.g., CaCO3), thereby removing the associated hardness—hence the term “temporary.” Permanent hardness is caused mainly by sulfates and chlorides of Ca and Mg that do not precipitate upon boiling and require chemical softening or ion exchange.

Step-by-Step Solution:

Verification / Alternative check:

Jar tests and classical water chemistry confirm that bicarbonate alkalinity correlates strongly with carbonate hardness; lime softening targets these species effectively.

Why Other Options Are Wrong:

Dissolved CO2 (A) affects pH but is not hardness; Na/K bicarbonates (C) do not contribute to hardness since Na+ and K+ are monovalent; option D ignores bicarbonates, the principal temporary hardness form; sulfates (E) cause permanent, not temporary, hardness.

Common Pitfalls:

Equating total alkalinity with temporary hardness without accounting for calcium/magnesium presence; overlooking magnesium’s contribution alongside calcium.

Final Answer:

bicarbonates and carbonates of calcium and magnesium