Question

Which substance is used for softening permanent hardness of water?

• A. Baking soda
• B. Washing soda
• C. Bleaching powder
• D. Common salt

Hint:

Important facts:
• Temporary hardness can be removed by boiling.
• Permanent hardness requires chemical treatment.
• Washing soda removes permanent hardness effectively.

Answer 1

The Correct Option is B

Concept:
Permanent hardness of water is caused by dissolved salts of:
• Calcium
• Magnesium especially chlorides and sulphates. This hardness cannot be removed simply by boiling. To remove permanent hardness, chemicals like washing soda are used.

Step 1: Understand washing soda.
Washing soda is: \[ Na_2CO_3 \cdot 10H_2O \] It reacts with calcium and magnesium salts to form insoluble carbonates which precipitate out.

Step 2: Understand why washing soda softens water.
Example reaction: \[ CaSO_4 + Na_2CO_3 \rightarrow CaCO_3 \downarrow + Na_2SO_4 \] The insoluble calcium carbonate settles down, thereby removing hardness.

Step 3: Analyze all options carefully.
Option (A): Baking soda
Used in cooking and baking. Not mainly used for water softening. Incorrect. Option (B): Washing soda
Used for removing permanent hardness. Correct. Option (C): Bleaching powder
Used for disinfection and bleaching. Incorrect. Option (D): Common salt
Used as table salt. Incorrect.

Step 4: Choose the correct answer.
Thus, the substance used for softening permanent hardness of water is: \[ \boxed{\text{Washing soda}} \] Hence, \[ \boxed{(2)\ \text{Washing soda}} \]