Which of the following compounds is used for water softening ?

Options

A. `Ca_(3)(PO_(4))_(2)`
B. `Na_(2)PO_(4)`
C. `Na_(6)P_(6)O_(18)`
D. `Na_(2)HPO_(4)`

Explanation

To determine which compound is used for water softening, we can analyze the options provided and identify the one that is known for this purpose.

### Step-by-Step Solution:

1. **Understanding Water Hardness**:
- Water hardness is primarily caused by the presence of dissolved calcium (Ca²⁺) and magnesium (Mg²⁺) ions. These ions can interfere with the effectiveness of soaps and detergents.

2. **Identifying Water Softening Agents**:
- Water softening agents are compounds that can remove or neutralize these hardness-causing ions. Commonly used compounds include sodium hexametaphosphate, sodium carbonate, and zeolites.

3. **Analyzing the Given Compound**:
- The compound mentioned in the question is sodium hexametaphosphate, which has the chemical formula Na6P6O18. It is commercially known as Calgon.

4. **Mechanism of Action**:
- Sodium hexametaphosphate works by reacting with calcium and magnesium ions in hard water. It forms inactive complex anions that prevent these ions from precipitating out when soap is added, thus softening the water.

5. **Conclusion**:
- Based on the information, sodium hexametaphosphate (Na6P6O18) is indeed used for water softening. Therefore, the correct answer is option C: Na6P6O18.

### Final Answer:
The compound used for water softening is sodium hexametaphosphate (Na6P6O18), also known as Calgon.

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