Which of the following ions will cause hardness in water sample?

Options

A. `Ca^(2+)`
B. `Na^(+)`
C. `Cl^(-)`
D. `K^(+)`

Explanation

To determine which ions cause hardness in a water sample, we need to understand what constitutes hard water. Hardness in water is primarily due to the presence of certain metal ions, particularly calcium (Ca²⁺) and magnesium (Mg²⁺). These ions can form insoluble precipitates with soap, leading to the hardness of water.

### Step-by-Step Solution:

1. **Identify the Ions**: Look at the list of ions provided in the question. Common ions that contribute to water hardness include calcium (Ca²⁺) and magnesium (Mg²⁺).

2. **Understand Water Hardness**: Water hardness is mainly caused by the presence of dissolved salts of calcium and magnesium. These can be in the form of:
- Calcium bicarbonate (Ca(HCO₃)₂)
- Magnesium bicarbonate (Mg(HCO₃)₂)
- Calcium sulfate (CaSO₄)
- Magnesium sulfate (MgSO₄)
- Calcium chloride (CaCl₂)
- Magnesium chloride (MgCl₂)

3. **Evaluate the Ions**: From the information provided, we know that:
- Calcium ions (Ca²⁺) contribute to hardness.
- Magnesium ions (Mg²⁺) also contribute to hardness.
- Other ions like sodium (Na⁺) and potassium (K⁺) do not contribute to hardness.

4. **Conclusion**: Since the question asks which ions cause hardness, we conclude that both calcium (Ca²⁺) and magnesium (Mg²⁺) ions are responsible. However, if the question specifies only one option and includes Ca²⁺, then that would be the correct answer.

5. **Final Answer**: Therefore, if the options include Ca²⁺, it will be the correct answer for causing hardness in water.