Reddish brown (chocolate) precipitate is formed by mixing solutions containing:

Options

A. `Cu^(2+) and [Fe(CN)_(6)]^(3-)` ions
B. `Cu^(2+) and [Fe(CN)_(6)]^(4-)`
C. `Pb^(2+) and SO_(4)^(2-)` ions
D. `Pb^(2+) and I^(-)` ions

Answer

To determine which solutions form a reddish brown (chocolate) precipitate, we need to analyze the given pairs of ions and their possible reactions.

### Step-by-Step Solution:

1. **Identify the Given Ions:**
The question provides four pairs of ions:
- Pair 1: Cu²⁺ and Fe(CN)₆³⁻
- Pair 2: Cu²⁺ and Fe(CN)₆⁴⁻
- Pair 3: Pb²⁺ and SO₄²⁻
- Pair 4: Pb²⁺ and I⁻

2. **Understand the Formation of Precipitates:**
A precipitate is formed when two soluble ions react to form an insoluble compound. The color of the precipitate can help identify the compound formed.

3. **Analyze Each Pair:**
- **Pair 1 (Cu²⁺ and Fe(CN)₆³⁻):** This combination does not typically form a reddish-brown precipitate.
- **Pair 2 (Cu²⁺ and Fe(CN)₆⁴⁻):** This combination is known to form a reddish-brown precipitate, specifically copper(II) ferrocyanide (Cu₂[Fe(CN)₆]).
- **Pair 3 (Pb²⁺ and SO₄²⁻):** This combination forms lead(II) sulfate (PbSO₄), which is a white precipitate.
- **Pair 4 (Pb²⁺ and I⁻):** This combination forms lead(II) iodide (PbI₂), which is a yellow precipitate.

4. **Conclusion:**
The reddish brown (chocolate) precipitate is specifically formed by the reaction of copper(II) ions (Cu²⁺) with ferrocyanide ions (Fe(CN)₆⁴⁻). Therefore, the correct answer is the second pair: Cu²⁺ and Fe(CN)₆⁴⁻.

### Final Answer:
The reddish brown (chocolate) precipitate is formed by mixing solutions containing Cu²⁺ and Fe(CN)₆⁴⁻.

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