Which of the following cannot be used as a test for `H_(2)O_(2)`?

Options

A. A paper dipped in Pbs (black) true white when brought in contact with `H_(2)O_(2)`.
B. It liberates iodine from KI solution which gives blue color with starch solution.
C. It gives blue color with `K_(4)[Fe(CN)_(6)]`.
D. It decolourises acidified `KMnO_(4)` solution.

Explanation

To determine which of the following cannot be used as a test for hydrogen peroxide (H₂O₂), we will analyze each option provided in the question.

### Step-by-Step Solution:

1. **Understanding the Tests for H₂O₂**:
- Hydrogen peroxide can be tested using various reagents that react with it to produce observable changes.

2. **Option A: Paper dipped in PbS**:
- When lead sulfide (PbS) comes into contact with H₂O₂, it gets oxidized to lead sulfate (PbSO₄), which is a white precipitate. This indicates a positive test for H₂O₂.
- **Conclusion**: This can be used as a test for H₂O₂.

3. **Option B: Liberating iodine gas**:
- When potassium iodide (KI) solution is added to H₂O₂, it reacts to liberate iodine gas (I₂), which is violet in color. This reaction can be used to indicate the presence of H₂O₂.
- **Conclusion**: This can also be used as a test for H₂O₂.

4. **Option C: K₄[Fe(CN)₆] in acidic medium**:
- The reaction of potassium ferrocyanide (K₄[Fe(CN)₆]) with H₂O₂ in an acidic medium produces potassium ferricyanide (K₃[Fe(CN)₆]), which is a bright red-colored precipitate. The expected test for H₂O₂ typically involves the formation of a blue-colored complex, not a red one.
- **Conclusion**: This is not a valid test for H₂O₂.

5. **Option D: Decolorizing acidified KMnO₄**:
- Acidified potassium permanganate (KMnO₄) is purple in color, and when it reacts with H₂O₂, it gets reduced to a colorless manganese(II) ion, effectively decolorizing the solution. This is a well-known test for H₂O₂.
- **Conclusion**: This can be used as a test for H₂O₂.

### Final Answer:
The option that **cannot be used as a test for H₂O₂** is **Option C**: K₄[Fe(CN)₆] in acidic medium, as it produces a bright red precipitate instead of the expected blue complex.

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