Hard water forms scum with soap, due to the presence of

Options

A. `CH_(3),COOH`
B. `NaOH`
C. `Ca(OH)_(2)`
D. `CaCl_(2)`

Explanation

**Step-by-Step Text Solution:**

1. **Understanding Hard Water**: Hard water is water that contains high concentrations of dissolved minerals, primarily calcium (Ca²⁺) and magnesium (Mg²⁺) ions. These minerals can come from various sources, such as limestone or mineral deposits.

2. **Formation of Scum**: When soap is added to hard water, it reacts with the calcium and magnesium ions present in the water. This reaction leads to the formation of insoluble compounds, which precipitate out of the solution as a solid. This solid is known as scum.

3. **Identifying the Cause of Scum**: The scum formation is specifically due to the presence of calcium and magnesium salts in the water. Common salts that contribute to hard water include:
- Calcium carbonate (CaCO₃)
- Calcium chloride (CaCl₂)
- Calcium sulfate (CaSO₄)
- Magnesium salts (like MgSO₄, MgCl₂)

4. **Analyzing the Options**: The question provides several options to identify the cause of scum formation:
- Option 1: CS₃COOH (Acetic acid) – This is an acid and does not contribute to scum formation.
- Option 2: NaOH (Sodium hydroxide) – This is a strong base and does not contribute to scum formation.
- Option 3: Ca(OH)₂ (Calcium hydroxide) – This is a base and does not directly cause scum formation.
- Option 4: CaCl₂ (Calcium chloride) – This is a salt that contributes to the hardness of water.

5. **Conclusion**: The presence of calcium chloride (CaCl₂) in water is responsible for the formation of scum when soap is used. Therefore, the correct answer is option 4: Calcium chloride.

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