Question

Hardness of water is due to the presence of salts of

• A. Potassium
• B. Chlorine
• C. Magnesium
• D. Boron

Hint:

Hardness = $Ca^{2+}$ + $Mg^{2+}$. While both contribute, magnesium is the specific option provided here. In aquaculture, we often express hardness in terms of equivalent milligrams per liter of Calcium Carbonate ($CaCO_3$).

Answer 1

The Correct Option is C

Step 1: Understanding the Question:
The question asks to identify the chemical element whose dissolved salts contribute to the "hardness" of water, a critical parameter in aquatic chemistry and aquaculture.

Step 2: Detailed Explanation:

• Definition of Water Hardness: Total hardness is defined as the sum of the concentrations of multivalent metal cations in the water. In most natural waters, the hardness is almost entirely due to the presence of Calcium ($Ca^{2+}$) and Magnesium ($Mg^{2+}$) ions.

• Types of Hardness:
- Temporary Hardness: Due to carbonates and bicarbonates of calcium and magnesium. It can be removed by boiling.
- Permanent Hardness: Due to sulfates, chlorides, and nitrates of calcium and magnesium. It cannot be removed by boiling.

• Importance in Aquaculture: Moderate hardness (usually 75-150 mg/L as $CaCO_3$) is beneficial for fish. Calcium is vital for bone development and osmoregulation, while magnesium is a cofactor for many enzymatic reactions. High hardness can reduce the toxicity of some heavy metals to fish.

• Analysis of other options: Potassium, Chlorine, and Boron do not contribute significantly to the property defined as "water hardness."

Step 3: Final Answer:
Water hardness is primarily due to the presence of Magnesium (and Calcium) salts.