Which of the following combinations in an aqueous medium will give a blue colour or precipitate ?

Options

A. `Fe^(2+)+[Fe(CN)_(6)]^(3+)`
B. `Fe^(3+)+[Fe(CN)_(6)]^(4-)`
C. `Hg^(2+)+SCN^(-)+Co^(2+)`
D. `Fe^(3+)+SCN^(-)`

Answer

To determine which combinations in an aqueous medium will give a blue color or precipitate, we will analyze each combination provided in the question.

### Step-by-Step Solution:

1. **Combination A: Fe²⁺ + [Fe(CN)₆]³⁻**
- When ferrous ion (Fe²⁺) reacts with ferricyanide ion ([Fe(CN)₆]³⁻), it forms a complex known as Turnbull's Blue, which is represented as Fe₃[Fe(CN)₆]₂. This complex exhibits a blue color.
- **Conclusion**: This combination gives a blue color precipitate.

2. **Combination B: Fe³⁺ + [Fe(CN)₆]⁴⁻**
- When ferric ion (Fe³⁺) reacts with ferrocyanide ion ([Fe(CN)₆]⁴⁻), it forms a complex known as Prussian Blue, represented as Fe₄[Fe(CN)₆]₃. This complex also exhibits a dark blue color.
- **Conclusion**: This combination also gives a blue color precipitate.

3. **Combination C: Hg²⁺ + CN⁻ + CO²⁺**
- In this case, the combination of mercuric ion (Hg²⁺), cyanide ion (CN⁻), and carbon dioxide (CO²⁺) forms a complex that is blue in color. The specific complex formed is Hg(CN)₂, which can exhibit blue color characteristics.
- **Conclusion**: This combination gives a blue color complex.

4. **Combination D: Fe³⁺ + SCN⁻**
- When ferric ion (Fe³⁺) reacts with thiocyanate ion (SCN⁻), it forms the complex Fe(SCN)₃, which is known for its blood-red color.
- **Conclusion**: This combination does not give a blue color; instead, it gives a blood-red color.

### Final Conclusion:
The combinations that will give a blue color or precipitate are:
- **Combination A**: Fe²⁺ + [Fe(CN)₆]³⁻ (Turnbull's Blue)
- **Combination B**: Fe³⁺ + [Fe(CN)₆]⁴⁻ (Prussian Blue)
- **Combination C**: Hg²⁺ + CN⁻ + CO²⁺ (blue complex)

Thus, the answer is **Combinations A, B, and C**.