Which of the following does not cause hardness of water ?

Options

A. `CaCl_(2)`
B. `MgSO_(4)`
C. `Na_(2)SO_(4)`
D. `FeSO_(4)`

Explanation

To determine which of the given compounds does not cause hardness of water, we need to analyze each option based on its solubility and its contribution to water hardness.

### Step-by-Step Solution:

1. **Identify the Compounds**: The options given are:
- Calcium chloride (CaCl₂)
- Magnesium sulfate (MgSO₄)
- Sodium sulfate (Na₂SO₄)
- Ferrous sulfate (FeSO₄)

2. **Understand Water Hardness**: Water hardness is primarily caused by the presence of dissolved minerals, specifically calcium (Ca²⁺) and magnesium (Mg²⁺) ions. These ions typically come from soluble salts.

3. **Analyze Each Compound**:
- **Calcium Chloride (CaCl₂)**: This compound dissociates in water to release calcium ions (Ca²⁺). Since calcium contributes to hardness, CaCl₂ causes hardness of water.
- **Magnesium Sulfate (MgSO₄)**: This compound dissociates in water to release magnesium ions (Mg²⁺). Magnesium also contributes to hardness, so MgSO₄ causes hardness of water.
- **Sodium Sulfate (Na₂SO₄)**: This compound dissociates in water to release sodium ions (Na⁺) and sulfate ions (SO₄²⁻). Sodium does not contribute to hardness, and Na₂SO₄ is not known for causing hardness in water.
- **Ferrous Sulfate (FeSO₄)**: This compound dissociates in water to release ferrous ions (Fe²⁺). Ferrous ions can contribute to hardness, so FeSO₄ causes hardness of water.

4. **Conclusion**: Among the four options, sodium sulfate (Na₂SO₄) does not cause hardness of water because it does not release calcium or magnesium ions.

### Final Answer:
**Sodium sulfate (Na₂SO₄) does not cause hardness of water.**

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