Question

Permanent hardness cannot be removed by

• A. Using washing soda
• B. Calgon's method
• C. Clark's method
• D. Ion exchange method

Answer 1

Answer: (C)

Clark's method

Answer 2

Clark's method is used for the removal of temporary hardness of water.

In Clark's process, slaked lime, Ca(OH)2​ is added to the hard water. Insoluble calcium carbonate precipitates out and no longer produces hardness.

• Ca(HCO 3​)2​ (aq) + Ca(OH)2 ​(aq) → 2CaCO3 ​(s) + 2H2​O (l)
• Mg(HCO 3​)2 ​(aq) + Ca(OH)2 ​(aq) → CaCO3 ​(s) + MgCO3 ​(s) + 2H2O (l)

Slaked lime is itself a source of calciumions and therefore of hardness, so care must be taken to avoid adding an excess of it.