`H_(2)O` is hard if it contains

Options

A. `NaHCO_(3)`
B. `MgSO_(4)`
C. KCl
D. NaCl

Explanation

To determine the conditions under which \( H_2O \) (water) is considered hard, we need to analyze the components that contribute to water hardness.

### Step-by-Step Solution:

1. **Definition of Hard Water**:
Hard water is defined as water that contains high concentrations of dissolved minerals, primarily calcium (\( Ca^{2+} \)) and magnesium (\( Mg^{2+} \)) ions.

2. **Types of Hardness**:
- **Temporary Hardness**: This type of hardness is caused by the presence of bicarbonate ions (\( HCO_3^- \)). It can be removed by boiling the water, which precipitates the calcium and magnesium as carbonates.
- **Permanent Hardness**: This type of hardness is due to the presence of sulfate (\( SO_4^{2-} \)) or chloride (\( Cl^- \)) ions. This hardness cannot be removed by boiling.

3. **Identifying Hard Water Components**:
For water to be classified as hard, it must contain:
- Calcium ions (\( Ca^{2+} \))
- Magnesium ions (\( Mg^{2+} \))
These ions can be present in the form of bicarbonates, sulfates, or chlorides.

4. **Analyzing the Options**:
- **Sodium Bicarbonate (\( NaHCO_3 \))**: Does not contribute to hardness as it does not contain \( Ca^{2+} \) or \( Mg^{2+} \).
- **Magnesium Sulfate (\( MgSO_4 \))**: Contributes to permanent hardness because it contains \( Mg^{2+} \).
- **Potassium Chloride (\( KCl \))**: Does not contribute to hardness as it does not contain \( Ca^{2+} \) or \( Mg^{2+} \).
- **Sodium Chloride (\( NaCl \))**: Does not contribute to hardness as it does not contain \( Ca^{2+} \) or \( Mg^{2+} \).

5. **Conclusion**:
Therefore, \( H_2O \) is hard if it contains calcium (\( Ca^{2+} \)) and/or magnesium (\( Mg^{2+} \)) ions, particularly in the forms of bicarbonates, sulfates, or chlorides.

### Final Answer:
\( H_2O \) is hard if it contains calcium (\( Ca^{2+} \)) and/or magnesium (\( Mg^{2+} \)) ions.