Question

The method used to remove temporary hardness of water is:

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

Answer 1

The Correct Option is D

To determine the method used to remove temporary hardness of water, let's first understand the concept of temporary hardness and the methods used for water softening.

Temporary Hardness: This type of hardness in water is primarily due to the presence of dissolved bicarbonate minerals, such as calcium bicarbonate (\text{Ca(HCO}_3)_2) and magnesium bicarbonate (\text{Mg(HCO}_3)_2). Temporary hardness can be removed by boiling the water or by adding lime (calcium hydroxide).

Explanation of Methods:

1. Clark's Method: This method specifically targets the removal of temporary hardness. It involves the addition of slaked lime (\text{Ca(OH)}_2) to the water. This process precipitates out the bicarbonate ions as calcium carbonate (\text{CaCO}_3), which can then be filtered out, thus removing the temporary hardness.
2. Ion-Exchange Method: This method is suitable for removing permanent hardness by exchanging calcium and magnesium ions with sodium ions. It is not generally used for temporary hardness.
3. Synthetic Resins Method: These are cation-exchange resins used to soften water by exchanging resin-contained hydrogen ions with calcium and magnesium ions in water, which effectively removes permanent hardness.
4. Calgon's Method: This is another method for addressing permanent hardness. Calgon (sodium hexametaphosphate) works by sequestering calcium and magnesium ions.

Conclusion: Among the options provided, Clark's method is the correct choice for removing temporary hardness from water. This is because it directly addresses the bicarbonate ions causing temporary hardness through precipitation.