On adding conc. `H_2SO_4`to a salt, brown colouration was observed on the walls of the test tube, On heating, violet coloured vapours were evolved. The brown colouration is due to the formation of

Options

A. `l_(2)`(s)
B. l
C. `l_(3)^(-)`
D. Hl

Answer

To solve the question regarding the brown coloration observed upon adding concentrated sulfuric acid (H₂SO₄) to a salt, we need to analyze the chemical reactions involved and the species formed.

### Step-by-Step Solution:

1. **Identify the Salt**: The salt in question likely contains iodide ions (I⁻). This is inferred from the observation of violet vapors upon heating, which are characteristic of iodine (I₂) gas.

**Hint**: Think about common salts that contain iodide ions.

2. **Reaction with Concentrated H₂SO₄**: When concentrated sulfuric acid is added to iodide salts, it acts as an oxidizing agent. The iodide ions (I⁻) are oxidized to triiodide ions (I₃⁻), which is responsible for the brown coloration observed on the walls of the test tube.

**Hint**: Consider how concentrated sulfuric acid can oxidize halide ions.

3. **Formation of Brown Coloration**: The brown coloration is due to the formation of triiodide ions (I₃⁻). This occurs because the I⁻ ions react with the sulfuric acid, leading to the formation of I₃⁻ in solution.

**Hint**: Recall that I₃⁻ is a complex ion that gives a brown color in solution.

4. **Heating the Mixture**: Upon heating, the triiodide ions can further decompose, and iodine gas (I₂) is released, which is responsible for the violet vapors observed. The sulfuric acid reduces the iodide ions to iodine while itself being reduced in the process.

**Hint**: Think about the changes that occur when a compound is heated and how it can lead to the release of gases.

5. **Final Conclusion**: Therefore, the brown coloration observed is due to the formation of triiodide ions (I₃⁻), and the violet vapors evolved upon heating are due to the release of iodine gas (I₂).

### Final Answer:
The brown coloration is due to the formation of I₃⁻ (triiodide ions).