There is formation of blue colour when `(KI+` Starch ) is added to

Options

A. `HNO_(2)`
B. `CuSO_(4)`
C. `Cl_(2)`
D. `Cr_(2)O_(7)^(-2)//H^(+)`

Explanation

To determine the formation of a blue color when potassium iodide (KI) is added to starch, we need to understand the underlying chemical reactions involved. The blue color is a result of the formation of the iodine-starch complex.

### Step-by-Step Solution:

1. **Understanding the Reaction**:
- The blue color formation occurs when iodine (I2) interacts with starch. The starch forms a complex with iodine, which results in a blue color.

2. **Oxidation of Iodide Ion**:
- In the presence of an oxidizing agent, iodide ions (I-) from potassium iodide (KI) are oxidized to molecular iodine (I2).
- The general reaction can be represented as:
\[ 2 KI + Oxidizing \ Agent \rightarrow I_2 + 2 K^+ \]

3. **Identifying the Oxidizing Agent**:
- The question suggests that we need to identify which oxidizing agent can oxidize iodide ions to iodine.
- Common oxidizing agents include HNO3, Cl2, Cr2O7^2-, etc.

4. **Reactions with Different Oxidizing Agents**:
- **With HNO3**:
\[ 4 HNO_3 + 2 KI \rightarrow I_2 + 2 KNO_3 + 2 NO_2 + 2 H_2O \]
- Here, iodide ions are oxidized to iodine.

- **With Cl2**:
\[ Cl_2 + 2 KI \rightarrow 2 KCl + I_2 \]
- Chlorine oxidizes iodide to iodine.

- **With Cr2O7^2-**:
\[ Cr_2O_7^{2-} + 14 H^+ + 6 I^- \rightarrow 2 Cr^{3+} + 3 I_2 + 7 H_2O \]
- The dichromate ion also oxidizes iodide to iodine.

5. **Formation of Blue Color**:
- Once iodine (I2) is formed, it can react with starch to produce the characteristic blue color:
\[ I_2 + Starch \rightarrow Blue \ Complex \]

6. **Conclusion**:
- The formation of blue color occurs when any of the mentioned oxidizing agents (HNO3, Cl2, or Cr2O7^2-) oxidizes iodide ions to iodine, which then reacts with starch.

### Final Answer:
The blue color is formed when KI is added to a solution containing an oxidizing agent like HNO3, Cl2, or Cr2O7^2-.