Hard water does not contain `:-`

Options

A. `Ca^(2+)` with `HCO_(3)^(-)`
B. `Mg^(2+)` with `SO_(4)^(2-)`
C. `Mg^(2+)` with `Cl^(-)`
D. `Ca^(2+)` with `CO_(3)^(2-)`

Explanation

To solve the question "Hard water does not contain :-", we need to understand the composition of hard water and identify which of the given options is not present in it.

### Step-by-Step Solution:

1. **Definition of Hard Water**:
- Hard water is defined as water that has a high concentration of dissolved minerals, primarily calcium (Ca²⁺) and magnesium (Mg²⁺) ions.

2. **Common Ions in Hard Water**:
- The main ions found in hard water are:
- Calcium ions (Ca²⁺)
- Magnesium ions (Mg²⁺)
- These ions can be present in various forms such as:
- Carbonate (CO₃²⁻)
- Bicarbonate (HCO₃⁻)
- Sulfate (SO₄²⁻)
- Chloride (Cl⁻)

3. **Analyzing the Options**:
- We need to evaluate the options provided to determine which ion or compound is not present in hard water.
- The options are:
1. Ca²⁺ and HCO₃⁻
2. Mg²⁺ and SO₄²⁻
3. Mg²⁺ and Cl⁻
4. Ca²⁺ and CO₃²⁻

4. **Identifying the Incorrect Option**:
- **Option 1**: Ca²⁺ and HCO₃⁻ - These ions are present in hard water.
- **Option 2**: Mg²⁺ and SO₄²⁻ - These ions are also present in hard water.
- **Option 3**: Mg²⁺ and Cl⁻ - These ions are present as well.
- **Option 4**: Ca²⁺ and CO₃²⁻ - While Ca²⁺ is present, it is primarily found with HCO₃⁻ in hard water, and the carbonate ion (CO₃²⁻) is not typically found in significant amounts in hard water.

5. **Conclusion**:
- The correct answer is **Option 4: Ca²⁺ and CO₃²⁻**, as hard water does not contain significant amounts of carbonate ions.

### Final Answer:
Hard water does not contain: **Ca²⁺ and CO₃²⁻**.

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