Permanent hardness of water cannot be removed by which one of the following methods ?

Options

A. Treatment with washing soda
B. Calgon's method
C. Boiling
D. Ion exchange method

Explanation

To answer the question regarding the removal of permanent hardness of water, we need to understand the nature of permanent hardness and the methods available for its removal.

### Step-by-Step Solution:

1. **Understanding Permanent Hardness**:
- Permanent hardness of water is primarily caused by the presence of dissolved salts of calcium and magnesium, specifically their chlorides and sulfates. Unlike temporary hardness, which is mainly due to bicarbonates, permanent hardness does not get removed by boiling.

2. **Methods to Remove Permanent Hardness**:
- **Calgon's Method**: This method involves adding sodium hexametaphosphate (Calgon) to water, which helps to sequester calcium and magnesium ions, thus removing hardness.
- **Ion Exchange Method**: This method uses ion exchange resins to replace calcium and magnesium ions with sodium ions, effectively softening the water.
- **Washing Soda Method**: Adding washing soda (sodium carbonate) to hard water precipitates calcium and magnesium ions as insoluble carbonates, thereby removing hardness.

3. **Identifying the Incorrect Method**:
- **Boiling**: Boiling water is effective for removing temporary hardness (due to bicarbonates) but does not affect permanent hardness. Therefore, boiling is the method that cannot remove permanent hardness.

### Conclusion:
Permanent hardness of water cannot be removed by boiling.

### Answer:
Permanent hardness of water cannot be removed by boiling.

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