A which solid gives a green residue on beingi subjected to the cobalt nitrate test.on being warmed with concentrated `H_(2)SO_(4)`, the solid gives a brown gas, which evolves vigorously on the addition of Cu turningsl. The solid may be:

Options

A. `Zn(NO_(3))_(2)`
B. `Al(NO_(3))_(2)`
C. `ZnBr_(2)`
D. `Mg(NO_(3))_(2)`

Answer

To solve the problem systematically, we can break it down into the following steps:

### Step 1: Identify the Reaction with Cobalt Nitrate
The problem states that a white solid gives a green residue when subjected to the cobalt nitrate test. The cobalt nitrate test involves heating cobalt nitrate, which leads to the formation of cobalt oxide (CoO), nitrogen dioxide (NO2), and oxygen (O2). The green residue is formed when cobalt oxide reacts with certain metal oxides.

**Hint:** Remember that the green residue is specifically associated with zinc oxide (ZnO) when cobalt oxide is involved.

### Step 2: Determine Possible Solids
The green residue indicates that the solid is likely a zinc compound, as zinc oxide reacts with cobalt oxide to form a green compound known as Riemann's green. Therefore, the possible solids could be zinc nitrate (Zn(NO3)2) or zinc bromide (ZnBr2).

**Hint:** Focus on the metal ions that can produce a green residue with cobalt oxide.

### Step 3: Analyze the Reaction with Concentrated H2SO4
Next, we need to analyze what happens when the solid is warmed with concentrated sulfuric acid (H2SO4). Both zinc nitrate and zinc bromide will react with H2SO4, but they produce different gases:
- **Zinc Nitrate (Zn(NO3)2)**: When heated, it decomposes to produce nitrogen dioxide (NO2), which is a brown gas.
- **Zinc Bromide (ZnBr2)**: When heated, it produces bromine (Br2), which is also brown but behaves differently.

**Hint:** Consider the nature of the gases produced and their color.

### Step 4: Examine the Reaction with Copper Turnings
The problem states that the brown gas evolves vigorously upon the addition of copper turnings. This is a key point because the vigorous evolution of gas is characteristic of reactions involving nitrate ions. When copper is added to a nitrate solution, it can reduce the nitrate ions, leading to the production of nitrogen dioxide (NO2) gas.

**Hint:** Remember that the vigorous evolution of gas upon adding copper is indicative of a nitrate compound.

### Step 5: Conclusion
Based on the above analysis, the solid must be zinc nitrate (Zn(NO3)2). It produces a green residue with cobalt nitrate, generates nitrogen dioxide gas when warmed with concentrated sulfuric acid, and evolves vigorously when copper turnings are added.

**Final Answer:** The solid may be zinc nitrate (Zn(NO3)2).

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