Upon dipping a copper rod, the aqueous solution of the salt that can turn blue is :-

Options

A. `Ca(NO_3)_2`
B. `Mg (NO_3)_2`
C. `Zn (NO_3)_2`
D. `Ag NO_3`

Explanation

To solve the question "Upon dipping a copper rod, the aqueous solution of the salt that can turn blue is:", we can follow these steps:

### Step 1: Understand the Reactivity Series
The reactivity series is a list of metals arranged in order of decreasing reactivity. Metals that are more reactive can displace less reactive metals from their compounds.

**Hint:** Remember that metals above hydrogen in the reactivity series are more reactive than those below it.

### Step 2: Identify the Position of Copper in the Reactivity Series
Copper is located below hydrogen in the reactivity series, which means it is less reactive compared to metals like calcium, magnesium, and zinc.

**Hint:** Look at the reactivity series to determine where copper stands in relation to other metals.

### Step 3: Consider Possible Salts
The question asks for an aqueous solution of a salt that can turn blue when a copper rod is dipped into it. Common salts that may be considered include silver nitrate (AgNO3), copper sulfate (CuSO4), and others.

**Hint:** Think about the color changes associated with different metal salts when they react with copper.

### Step 4: Analyze the Reaction with Silver Nitrate (AgNO3)
When a copper rod is dipped in silver nitrate solution, copper can displace silver because it is less reactive than copper. The reaction produces copper nitrate (Cu(NO3)2), which is blue in color.

**Hint:** Recall that a color change indicates a chemical reaction has occurred.

### Step 5: Conclusion
The only salt solution that can turn blue upon dipping a copper rod is silver nitrate (AgNO3). The formation of copper nitrate in the solution gives it a blue color.

**Final Answer:** The aqueous solution of the salt that can turn blue is AgNO3.

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