In three test tubes A, B and C, three different liquids namely, distilled water, underground water and distilled water in which a pinch of calcium sulfate is dissolved, respectively are taken, Equal amount of soap solution is added to each test tube and the contents are shaken. In which test tube will the length of the foam (lather) be longest? Justify your answer.

Explanation

To determine which test tube will have the longest length of foam (lather) after adding soap solution, we need to analyze the contents of each test tube and how they interact with soap.

### Step-by-Step Solution:

1. **Identify the contents of each test tube:**
- Test Tube A: Distilled water (pure water, free from dissolved ions)
- Test Tube B: Underground water (contains various dissolved salts, including calcium and magnesium ions)
- Test Tube C: Distilled water with a pinch of calcium sulfate (contains calcium ions)
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