Which of the statements is not true ?

Options

A. (a) On passing `H_(2)S` through acidified `K_(2)Cr_(2)O_(7)` solution , a milky colour is observed
B. (b) `Na_(2)Cr_(2)O_(7)` is preferred over `K_(2)Cr_(2)O_(7)` in volumetric analysis
C. (c) `K_(2)Cr_(2)O_(7)` solution in acidic medium is orange
D. (d) `K_(2)Cr_(2)O_(7)` solution becomes yellow on increasing the pH beyond 7

Explanation

To solve the question regarding which statement is not true, we will analyze each option step by step.

### Step-by-Step Solution:

1. **Analyze Statement A**:
- **Statement**: "On passing H2S through acidified potassium dichromate solution a milky color is observed."
- **Explanation**: When H2S gas is passed through an acidified potassium dichromate solution, a reaction occurs that leads to the formation of sulfur, which appears as a colloidal suspension, giving the solution a milky appearance. This statement is **true**.

2. **Analyze Statement B**:
- **Statement**: "Sodium dichromate is preferred over potassium dichromate in volumetric analysis."
- **Explanation**: Sodium dichromate is hygroscopic, meaning it absorbs moisture from the air, making it difficult to prepare a standard solution accurately. In contrast, potassium dichromate is not hygroscopic, allowing for easier preparation of standard solutions. Therefore, this statement is **not true**.

3. **Analyze Statement C**:
- **Statement**: "Potassium dichromate solution is acidic, medium is orange."
- **Explanation**: In an acidic medium, potassium dichromate (K2Cr2O7) exhibits an orange color. This statement is **true**.

4. **Analyze Statement D**:
- **Statement**: "Potassium dichromate solution becomes yellow on increasing the pH beyond 7."
- **Explanation**: When the pH of potassium dichromate solution is increased (making it basic), the solution changes color from orange to yellow. This statement is **true**.

### Conclusion:
After analyzing all statements, we find that the statement which is **not true** is **Option B**: "Sodium dichromate is preferred over potassium dichromate in volumetric analysis."

### Final Answer:
**The statement that is not true is Option B.**

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