Which one of the following processes will produce permanent hard water ?

Options

A. Addition of `Na_(2)SO_(4)` to water
B. Saturation of water with `CaCO_(3)`
C. Saturation of water with `MgCO_(3)`
D. Saturation of water with `CaSO_(4)`

Answer

To determine which process will produce permanent hard water, we need to understand the types of hardness in water.

### Step-by-Step Solution:

1. **Understanding Hard Water**:
- Hard water contains dissolved minerals, primarily calcium (Ca²⁺) and magnesium (Mg²⁺) ions.
- Hardness is classified into two types:
- **Temporary Hardness**: Caused by the presence of bicarbonates (HCO₃⁻) and carbonates (CO₃²⁻) of calcium and magnesium. This type of hardness can be removed by boiling.
- **Permanent Hardness**: Caused by the presence of chlorides (Cl⁻) and sulfates (SO₄²⁻) of calcium and magnesium. This type of hardness cannot be removed by boiling.

2. **Analyzing the Options**:
- **Option A: Addition of Na₂SO₄ to water**: Sodium sulfate does not contribute to hardness, as it does not contain calcium or magnesium ions.
- **Option B: Saturation of water with CaCO₃**: Calcium carbonate contributes to temporary hardness, as it can be precipitated out by boiling.
- **Option C: Saturation of water with MgCO₃**: Magnesium carbonate also contributes to temporary hardness and can be removed by boiling.
- **Option D: Saturation of water with CaSO₄**: Calcium sulfate contributes to permanent hardness, as it introduces calcium ions in the form of sulfate, which cannot be removed by boiling.

3. **Conclusion**:
- The process that will produce permanent hard water is **saturation of water with CaSO₄** (Option D).

### Final Answer:
**Option D: Saturation of water with CaSO₄ will produce permanent hard water.**