A strip of copper is dipped into a colourless solution of the following four salts , which are placed separately in four different test tubes. Which solution will turn Blue ?

Options

A. `KNO_3`
B. `ZnSO_4`
C. `Zn(NO_3)_2`
D. `AgNO_3`

Answer

To determine which solution will turn blue when a strip of copper is dipped into a colorless solution of the given salts, we need to analyze the reactivity of copper with each salt solution.

### Step-by-Step Solution:

1. **Identify the Salts**: The four salts mentioned are potassium nitrate, zinc sulfate, zinc nitrate, and silver nitrate.

2. **Reactivity Series**: Recall the reactivity series of metals. In this series, metals are arranged according to their ability to displace other metals from solutions. The more reactive a metal is, the more likely it is to displace a less reactive metal.

3. **Potassium Nitrate**:
- When copper is placed in potassium nitrate (KNO₃), no reaction occurs because potassium (K) is more reactive than copper (Cu). Therefore, copper cannot displace potassium from the solution.
- **Conclusion**: No blue color change.

4. **Zinc Sulfate**:
- When copper is placed in zinc sulfate (ZnSO₄), again no reaction occurs because zinc (Zn) is more reactive than copper. Thus, copper cannot displace zinc.
- **Conclusion**: No blue color change.

5. **Zinc Nitrate**:
- Similarly, when copper is placed in zinc nitrate (Zn(NO₃)₂), no reaction occurs for the same reason as above; zinc is more reactive than copper.
- **Conclusion**: No blue color change.

6. **Silver Nitrate**:
- When copper is placed in silver nitrate (AgNO₃), a reaction occurs. Copper is more reactive than silver and can displace silver from the solution. The reaction can be represented as:
\[ \text{Cu (s)} + 2 \text{AgNO}_3 (aq) \rightarrow \text{Cu(NO}_3)_2 (aq) + 2 \text{Ag (s)} \]
- This reaction produces copper(II) nitrate, which is blue in color, and a grayish-white deposit of silver.
- **Conclusion**: The solution turns blue.

### Final Answer:
The solution that will turn blue when a strip of copper is dipped into it is **Silver Nitrate (AgNO₃)**.