Question

Permanent hardness in water cannot be cured by :

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

Answer 1

The Correct Option is A

To address the question on what does not cure permanent hardness in water, we need to understand the causes of permanent hardness and the methods used to treat it.

Permanent Hardness: This type of hardness is due to the presence of soluble salts of calcium and magnesium, primarily in the form of chlorides and sulfates (e.g., CaCl₂, MgSO₄).

Permanent hardness cannot be removed by boiling. Boiling only precipitates the bicarbonate ions responsible for temporary hardness and is ineffective against the chlorides and sulfates responsible for permanent hardness.

1. Boiling: Boiling can remove only temporary hardness by precipitating bicarbonates. It does not affect permanent hardness.
2. Ion Exchange Method: This method involves exchanging calcium and magnesium ions with sodium ions using a resin, effectively removing both temporary and permanent hardness.
3. Calgon Method: In this method, an agent called Calgon (sodium hexametaphosphate) is used to complex calcium ions, effectively preventing scale formation and removing hardness.
4. Treatment with Washing Soda: Washing soda (sodium carbonate) reacts with soluble calcium and magnesium salts to precipitate them as carbonates, removing hardness.

Given the options, Boiling cannot cure permanent hardness, making Boiling the correct answer.

The explanations highlight that boiling only addresses temporary hardness, while the other methods target both temporary and permanent hardness through ion exchange, chemical complexation, or precipitation.