What do carbonates (CO3^2−) and bicarbonates (HCO3^−) in water primarily produce with respect to water-quality characterization?

Introduction / Context:
Understanding the carbonate system in water is central to treating drinking water and controlling corrosion. Carbonates and bicarbonates influence pH buffering and are measured as alkalinity, affecting coagulant dose and stability.

Given Data / Assumptions:

• Presence of carbonate (CO3^2−), bicarbonate (HCO3^−), and hydroxide (OH^−) species.
• Alkalinity is expressed as CaCO3 equivalent.

Concept / Approach:
Alkalinity is the capacity of water to neutralize acids and is predominantly due to bicarbonates, carbonates, and hydroxides. While bicarbonates of calcium/magnesium are associated with temporary hardness, the direct characteristic produced by these anions is alkalinity.

Step-by-Step Solution:

Verification / Alternative check:
Titration to phenolphthalein and methyl orange endpoints quantifies carbonate and bicarbonate alkalinity.

Why Other Options Are Wrong:

• (a) Temporary hardness refers to Ca/Mg bicarbonates but the question asks what carbonates in water produce primarily—alkalinity is more accurate.
• (b) Permanent hardness arises from sulphates/chlorides of Ca/Mg.
• (c) Carbonates do not produce acidity; they buffer against it.

Common Pitfalls:
Equating bicarbonate presence directly with “temporary hardness” when the more fundamental descriptor for anions is alkalinity.

Final Answer:
Alkalinity