The reagent which is not used for removing the permanent hardness of water is `:`

Options

A. washing soda
B. sodium hexametaphophate
C. Glauber's salt
D. hydrated sodium aluminium silicate ( zeolite )

Explanation

To determine the reagent that is not used for removing the permanent hardness of water, we need to understand what causes permanent hardness and the methods used to remove it.

### Step-by-Step Solution:

1. **Understanding Permanent Hardness**:
- Permanent hardness of water is primarily due to the presence of dissolved minerals, specifically calcium ions (Ca²⁺) and magnesium ions (Mg²⁺), often in the form of chlorides (Cl⁻) and sulfates (SO₄²⁻).

2. **Identifying Reagents Used for Removal**:
- **Washing Soda (Sodium Carbonate, Na₂CO₃)**:
- When washing soda is added to hard water, it reacts with calcium ions to form insoluble calcium carbonate (CaCO₃), which precipitates out, thus removing hardness.
- Reaction:
\[ \text{CaCl}_2 + \text{Na}_2\text{CO}_3 \rightarrow \text{CaCO}_3 \downarrow + 2 \text{NaCl} \]

- **Sodium Hexametaphosphate (Na₆P₆O₁₈)**:
- This compound can also be used to remove hardness by forming soluble complexes with calcium and magnesium ions.
- Reaction:
\[ \text{CaCl}_2 + \text{Na}_6\text{P}_6\text{O}_{18} \rightarrow \text{Na}_2\text{Ca}_2\text{P}_6\text{O}_{18} + 4 \text{NaCl} \]

- **Zeolites (Hydrated Sodium Aluminum Silicate)**:
- Zeolites are used in water softening processes. They exchange sodium ions for calcium and magnesium ions, effectively removing hardness.
- Reaction:
\[ \text{Na}_2\text{Zeolite} + \text{MgCl}_2 \rightarrow \text{MgZe} + 2 \text{NaCl} \]

3. **Identifying the Incorrect Reagent**:
- **Global Salt (Sodium Sulfate, Na₂SO₄)**:
- This compound does not participate in the removal of permanent hardness. It does not react with calcium or magnesium ions to precipitate them or form soluble complexes.
- Therefore, it is the reagent that is not used for removing permanent hardness.

### Conclusion:
The reagent that is not used for removing the permanent hardness of water is **Global Salt (Sodium Sulfate, Na₂SO₄)**.

---