To solve this problem, we need to identify the method that cannot remove permanent hardness from water.
1. Understanding Hardness in Water:
Hardness in water is caused by the presence of dissolved minerals, mainly calcium (Ca²⁺) and magnesium (Mg²⁺) ions. There are two types of hardness:
- Temporary hardness: Caused by dissolved bicarbonates (Ca(HCO₃)₂, Mg(HCO₃)₂) that can be removed by boiling.
- Permanenent hardness: Caused by dissolved sulphates and chlorides (such as CaSO₄, MgSO₄) and cannot be removed by boiling. This requires special treatments like ion exchange, washing soda, or other methods.
2. Analyzing Each Method:
Let’s evaluate each option to see if it can remove permanent hardness:
- Option (A) "Using washing soda": Washing soda (Na₂CO₃) is commonly used to remove both temporary and permanent hardness. It reacts with the calcium and magnesium ions to form insoluble carbonates, effectively removing the hardness. This method works for both types of hardness.
- Option (B) "Calgon's method": Calgon (sodium hexametaphosphate) is effective in removing both temporary and permanent hardness by forming a soluble complex with calcium and magnesium ions, making them unavailable for causing hardness. This method can remove permanent hardness.
- Option (C) "Clark's method": Clark's method involves adding lime (Ca(OH)₂) to water to precipitate calcium as calcium carbonate, which removes permanent hardness. This is an effective method for removing permanent hardness.
- Option (D) "Ion exchange method": The ion exchange method uses resins to exchange calcium and magnesium ions with sodium ions, effectively removing both temporary and permanent hardness. This method is widely used in water softening.
Final Answer:
All the methods mentioned (washing soda, Calgon's method, Clark's method, and ion exchange) are capable of removing permanent hardness. Therefore, the answer is (C) "Clark's method," which is not the correct method as it only removes temporary hardness.