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 it, we need to analyze the possible reactions between copper and the cations present in the solutions of the salts.

### Step-by-Step Solution:

1. **Identify the Cations in the Solutions**:
The question mentions four different salts. The cations present in these salts are:
- K⁺ (Potassium)
- Zn²⁺ (Zinc)
- Ag⁺ (Silver)
- Cu²⁺ (Copper)

2. **Understand the Electrochemical Series**:
The electrochemical series ranks metals based on their ability to be oxidized (lose electrons). The order of the cations in terms of their reactivity (from most reactive to least reactive) is:
- K⁺ > Zn²⁺ > Cu²⁺ > Ag⁺

3. **Determine Possible Reactions**:
When copper is dipped into these solutions, it can potentially displace a less reactive metal ion from its salt. The relevant reactions to consider are:
- Copper (Cu) can displace Ag⁺ from AgNO₃ because Ag⁺ is less reactive than Cu.
- Copper cannot displace Zn²⁺ or K⁺ because they are more reactive than Cu.

4. **Reaction with Silver Nitrate (AgNO₃)**:
If copper is dipped into a solution of AgNO₃, the following reaction will occur:
\[ Cu (s) + 2AgNO₃ (aq) \rightarrow 2Ag (s) + Cu(NO₃)₂ (aq) \]
This reaction produces solid silver (Ag) and copper(II) nitrate (Cu(NO₃)₂). The solution will turn blue due to the formation of Cu²⁺ ions in the solution.

5. **Conclusion**:
Therefore, the solution that will turn blue is the one containing silver nitrate (AgNO₃).

### Final Answer:
The solution that will turn blue is the one containing **AgNO₃**.