Question

Hardness is imparted to water mainly due to the presence of

• A. Divalent ions
• B. Divalent cations
• C. Monovalent cations
• D. Monovalent anions

Hint:

Water Hardness Facts: Total Hardness is caused primarily by DIVALENT CATIONS: $\text{Ca}^{2+}$ and $\text{Mg}^{2+}$. Optimum for fish culture = $75 - 150\text{ ppm as } \text{CaCO}_3$.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
Water chemistry in aquaculture: total hardness is defined as the sum of multivalent metallic cations in solution, predominantly divalent calcium ($ ext{Ca}^{2+}$) and magnesium ($ ext{Mg}^{2+}$) ions.
Key Formula or Approach:
\[ \text{Total Hardness (mg/L as } \text{CaCO}_3) = 2.497 [\text{Ca}^{2+}] + 4.118 [\text{Mg}^{2+}] \]

Step 2: Detailed Explanation:
In aquatic chemistry and aquaculture water quality engineering:
- Total Hardness is formally defined as the cumulative concentration of polyvalent/divalent metallic cations in water, expressed as equivalent milligrams per liter of Calcium Carbonate ($\text{mg/L as } \text{CaCO}_3$).
- It is primarily caused by Divalent Cations (B), principally:
1. Calcium ions ($\text{Ca}^{2+}$)
2. Magnesium ions ($\text{Mg}^{2+}$)
(with minor contributions from divalent $\text{Fe}^{2+}, \text{Mn}^{2+}, \text{Sr}^{2+}$).
- (Monovalent cations like $\text{Na}^+, \text{K}^+$ do not contribute to water hardness).

Step 3: Final Answer:
Therefore, hardness is imparted mainly due to Divalent cations, corresponding to option (B).