The pair of ions which does not cause hardness of water is

Options

A. sulphate, chloride
B. bicarbonate, chloride
C. bicarbonate, sulphate
D. nitrate, phosphate

Explanation

To determine the pair of ions that does not cause hardness of water, we need to understand what causes hardness in water and which ions are responsible for it.

### Step-by-Step Solution:

1. **Understand Hard Water**:
- Hard water is water that does not form lather with soap easily. This is primarily due to the presence of certain ions.

2. **Identify Common Ions Causing Hardness**:
- The main ions that contribute to water hardness are:
- Calcium ions (Ca²⁺)
- Magnesium ions (Mg²⁺)
- Bicarbonate ions (HCO₃⁻)
- Sulfate ions (SO₄²⁻)

3. **Identify Ions That Do Not Cause Hardness**:
- Ions that do not contribute to hardness include:
- Nitrate ions (NO₃⁻)
- Phosphate ions (PO₄³⁻)
- Fluoride ions (F⁻)

4. **Conclusion**:
- From the information above, the pair of ions that does not cause hardness of water is **nitrate (NO₃⁻) and phosphate (PO₄³⁻)**.

### Final Answer:
The pair of ions which does not cause hardness of water is **nitrate (NO₃⁻) and phosphate (PO₄³⁻)**.