Question

Which of the following does not cause the permanent hardness in water?

• A. Nitrite
• B. Nitrates
• C. Chlorides
• D. Bicarbonates

Hint:

Remember:
- Temporary Hardness = Bicarbonates (removed by boiling).
- Permanent Hardness = Sulfates, Chlorides, and Nitrates (not removed by boiling).

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
Water hardness is caused by the presence of dissolved multivalent metallic cations, primarily calcium (\( \text{Ca}^{2+} \)) and magnesium (\( \text{Mg}^{2+} \)).
Hardness is classified into two types:
1. Temporary hardness: Caused by the presence of dissolved bicarbonate minerals of calcium and magnesium. It can be removed by boiling the water.
2. Permanent hardness: Caused by the presence of sulfate, chloride, and nitrate salts of calcium and magnesium. It cannot be removed by boiling.

Step 2: Detailed Explanation:
Let us analyze how each option affects water hardness:
- Nitrite (\( \text{NO}_2^- \)):
An intermediate product in the nitrogen cycle that does not directly contribute to the mineral hardness of water.
- Nitrates (\( \text{NO}_3^- \)) and Chlorides (\( \text{Cl}^- \)):
When these anions associate with calcium or magnesium ions, they form highly soluble salts (such as calcium chloride or magnesium nitrate) that cause permanent hardness in water.
Boiling does not precipitate these salts, and removing them requires chemical treatments like ion exchange or reverse osmosis.
- Bicarbonates (\( \text{HCO}_3^- \)):
Calcium and magnesium bicarbonates cause temporary hardness.
When water containing these minerals is boiled, the soluble bicarbonates decompose into insoluble carbonates, water, and carbon dioxide:
\[ \text{Ca(HCO}_3)_2 \rightarrow \text{CaCO}_3 \downarrow + \text{ H}_2\text{O} + \text{CO}_2 \uparrow \]
The insoluble calcium carbonate (\( \text{CaCO}_3 \)) precipitates out of solution, reducing the hardness of the water.
Therefore, bicarbonates do not cause permanent hardness.

Step 3: Final Answer:
Bicarbonates cause temporary hardness in water, which can be removed by boiling, and do not cause permanent hardness.
Thus, the correct choice is (D).