Question

The method used to remove temporary hardness of water is : (1) (2) (3) (4)

• A. Calgon's method
• B. Clark's method
• C. Ion-exchange method
• D. Synthetic resins method

Answer 1

The Correct Option is B

To understand how to remove temporary hardness of water, let's first examine what temporary hardness is. Temporary hardness is primarily caused by the presence of dissolved bicarbonate minerals, typically calcium bicarbonate (\text{Ca(HCO}_3\text{)}_2) and magnesium bicarbonate (\text{Mg(HCO}_3\text{)}_2) in water. When water is heated, bicarbonates precipitate as carbonates and release carbon dioxide and water, enabling the hardness to be removed by simple boiling. However, more efficient methods exist for larger scale or specific applications. One such method is Clark's method.

Clark's Method: Clark's method involves adding slaked lime (\text{Ca(OH)}_2) to the water. The lime reacts with the bicarbonate ions, resulting in the precipitation of calcium carbonate (\text{CaCO}_3), which can be filtered out. This process is effective in removing the temporary hardness:

The chemical reaction can be represented as:

\text{Ca(HCO}_3\text{)}_2 + \text{Ca(OH)}_2 \rightarrow 2\text{CaCO}_3 \downarrow + 2\text{H}_2\text{O}

In this reaction, the slaked lime reacts with the calcium bicarbonate in the water, producing calcium carbonate precipitate and water. This helps in removing temporary hardness effectively.

Comparing with Other Methods:

• Calgon's method: This method is used for softening water by removing calcium and magnesium ions, and is more suitable for permanent hardness.
• Ion-exchange method: This involves exchanging calcium and magnesium ions with sodium ions, effective for both temporary and permanent hardness, but generally more expensive.
• Synthetic resins method: Similar to the ion-exchange method, this is used for more extensive treatment of water hardness.

Among the given options, Clark's method is explicitly designed to target temporary hardness by precipitating the bicarbonate compounds as carbonates, making it the correct and specific choice for this question.