The principal cause of hardness of water is the presence of_____ and____ions.

Explanation

To solve the question regarding the principal cause of hardness of water, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Hard Water**:
- Hard water is defined as water that does not produce lather with soap. This characteristic is due to the presence of certain ions in the water.

2. **Identifying the Ions**:
- The primary ions responsible for water hardness are calcium ions (Ca²⁺) and magnesium ions (Mg²⁺). These ions can form insoluble salts with soap, which is why hard water does not lather well.

3. **Types of Hardness**:
- There are two types of hardness:
- **Temporary Hardness**: Caused by bicarbonates of calcium and magnesium. This type of hardness can be removed by boiling the water.
- **Permanent Hardness**: Caused by chlorides and sulfates of calcium and magnesium. This type cannot be removed by boiling and requires specific treatment methods.

4. **Conclusion**:
- Therefore, the principal cause of hardness of water is the presence of calcium ions (Ca²⁺) and magnesium ions (Mg²⁺).

### Final Answer:
The principal cause of hardness of water is the presence of **calcium ions (Ca²⁺)** and **magnesium ions (Mg²⁺)**.

---