`NO_(3)^(Θ)` is detected by (a) _____when (b)_____is formed on the addition of `FeSO_(4)` and cone `H_(2)SO_(4)`

Answer

To solve the question regarding the detection of nitrate ions (NO₃⁻) using ferrous sulfate (FeSO₄) and concentrated sulfuric acid (H₂SO₄), we can break it down into the following steps:

### Step-by-Step Solution:

1. **Understanding the Detection of Nitrate Ions**:
- Nitrate ions (NO₃⁻) can be detected using a specific test known as the "ring test". This test is commonly used for identifying the presence of nitrate ions in a sample.

2. **Performing the Ring Test**:
- In the ring test, a small amount of ferrous sulfate (FeSO₄) is added to the solution containing the nitrate ions.

3. **Adding Concentrated Sulfuric Acid**:
- After adding the ferrous sulfate, concentrated sulfuric acid (H₂SO₄) is carefully added to the mixture. It is important to add the acid slowly and carefully to avoid any violent reactions.

4. **Observation of the Reaction**:
- Upon the addition of concentrated sulfuric acid, a brown ring is formed at the interface of the two liquids. This brown ring is indicative of the presence of nitrous acid (HNO₂) which is formed in situ from the reaction of nitrate ions with ferrous ions.

5. **Formation of the Compound**:
- The compound that is formed during this reaction is known as "ferrous nitrate" or "iron(II) nitrate", which can be represented as Fe(H₂O)₃NO₂⁻. This compound is responsible for the characteristic brown ring observed in the test.

### Final Answer:
(a) **Ring test**
(b) **Fe(H₂O)₃NO₂⁻**