Assertion : Soft water lathers with soap but not hard water.
Reason : Hard water reacts with soap to form insoluble salts which form scum, not lather.

Options

A. If both assertion and reason are true and reason is the correct explanation of assertion.
B. If both assertion and reason are true but reason is not the correct explanation of assertion.
C. If assertion is true but reason is false.
D. If both assertion and reason are false.

Answer

### Step-by-Step Solution:

1. **Understanding the Assertion**:
- The assertion states that "soft water lathers with soap but not hard water."
- This means that when soap is added to soft water, it forms lather, while in hard water, it does not.

2. **Understanding the Reason**:
- The reason provided is that "hard water reacts with soap to form insoluble salts which form scum, not lather."
- This indicates that the presence of certain ions in hard water (like calcium and magnesium) leads to a reaction with soap, resulting in the formation of insoluble compounds.

3. **Identifying the Components**:
- Soap is typically sodium stearate, which has the chemical formula C17H35COONa.
- Hard water contains calcium ions (Ca²⁺) and magnesium ions (Mg²⁺).

4. **Chemical Reaction**:
- When soap reacts with calcium ions, the reaction can be represented as:
\[ \text{R-C(=O)O-Na} + \text{Ca}^{2+} \rightarrow \text{R-C(=O)O-Ca} + \text{Na}^+ \]
- This results in the formation of calcium stearate, which is an insoluble salt.
- A similar reaction occurs with magnesium ions:
\[ \text{R-C(=O)O-Na} + \text{Mg}^{2+} \rightarrow \text{R-C(=O)O-Mg} + \text{Na}^+ \]
- This results in the formation of magnesium stearate, which is also insoluble.

5. **Conclusion**:
- Since both calcium stearate and magnesium stearate are insoluble, they precipitate out of the solution and form scum instead of lather.
- Therefore, the assertion is true, and the reason correctly explains why hard water does not lather with soap.

6. **Final Statement**:
- Both the assertion and the reason are correct, and the reason provided is the correct explanation for the assertion.