In the laboratory of a school , the sample of hard water is not available " to study the comparative cleaning capacity of a sample of soap in soft and hard water". Which salt from the laboratory can be added to tap water to make it hard ? In the experiment how is cleaning capacity of soap compared ?

Explanation

### Step-by-Step Solution:

1. **Identify the Salts Responsible for Hardness**:
- Hard water contains dissolved minerals, primarily calcium (Ca²⁺) and magnesium (Mg²⁺) ions. These ions are responsible for the hardness of water.

2. **Choose a Salt to Add to Tap Water**:
- To make tap water hard, you can add salts such as calcium chloride (CaCl₂) or magnesium sulfate (MgSO₄). These salts will introduce calcium and magnesium ions into the water, simulating hard water conditions.

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