A student takes about 5 mL distilled water in four test tubes marked P, Q, R and S. He dissolves sodium chloride in P, potassium chloride in Q, calcium chloride in R and magnesium chloride in S. After that he adds equal amount of soap solution in each test tube. On shaking the solution of each test tubes he would observe a good amount of lather in the test tubes marked

Options

A. P and Q
B. Q and R
C. R and S
D. P and S

Explanation

To solve the problem, we need to analyze the solubility of soap in the solutions created in each test tube after adding the respective salts.

### Step-by-Step Solution:

1. **Identify the Solutions in Each Test Tube:**
- Test tube P: Contains sodium chloride (NaCl).
- Test tube Q: Contains potassium chloride (KCl).
- Test tube R: Contains calcium chloride (CaCl₂).
- Test tube S: Contains magnesium chloride (MgCl₂).

2. **Understand the Effect of Ions on Soap:**
- Soap is a sodium or potassium salt of fatty acids and works effectively in soft water.
- Hard water contains divalent ions such as Ca²⁺ (calcium) and Mg²⁺ (magnesium), which react with soap to form insoluble precipitates, reducing the amount of lather produced.

3. **Analyze Each Test Tube:**
- **Test Tube P (NaCl):** Sodium ions (Na⁺) do not cause hardness. Therefore, soap will lather well.
- **Test Tube Q (KCl):** Potassium ions (K⁺) also do not cause hardness. Therefore, soap will lather well.
- **Test Tube R (CaCl₂):** Calcium ions (Ca²⁺) cause hardness. Soap will not lather well.
- **Test Tube S (MgCl₂):** Magnesium ions (Mg²⁺) cause hardness. Soap will not lather well.

4. **Conclusion:**
- The test tubes that will produce a good amount of lather are P and Q, as they contain sodium and potassium ions, which do not interfere with soap's ability to lather.

### Final Answer:
The test tubes marked P and Q will observe a good amount of lather.

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