Iron (+II) is one of the most important oxidation states and salts are called ferrous salts. Most of the `Fe(+II)` salts are pale green and contain `[Fe(H_(2)O)_(6_]^(2+)` ion. Fe(+II) compounds are easily oxidised by air and so are difficult to obtain pure `Fe^(2+)` form many complexes like `K_(4)[Fe(CN)_(6)]`.
Q.`FeSO_(4)` is used in brown ring test for nitrates and nitrites. In this test, a freshly prepared `FeSO_(4)` solution is mixed with solution containing `NO_(2)^(-) or NO_(3)^(-)` and the conc. `H_(2)SO_(4)` is run down the side of the test tube. It the mixture gets hot or is shaken.
(I) the brown colour disappear (II)NO is evolved (III) a yellow solution in `Fe_(2)(SO_(4))_(3)` is formed

Options

A. I, II, III
B. I, III
C. II, III
D. I only

Answer

To solve the problem regarding the brown ring test for nitrates and nitrites using `FeSO4`, we will break down the process step by step.

### Step-by-Step Solution:

1. **Understanding the Brown Ring Test**:
- The brown ring test is used to detect the presence of nitrates (NO3-) and nitrites (NO2-) in a solution. A freshly prepared solution of ferrous sulfate (FeSO4) is mixed with the solution containing nitrates or nitrites.

2. **Preparation of the Mixture**:
- A freshly prepared `FeSO4` solution is mixed with a solution containing `NO2^-` or `NO3^-`. Concentrated sulfuric acid (H2SO4) is then carefully added down the side of the test tube to avoid mixing.

3. **Formation of the Brown Ring**:
- When the two layers mix, a brown ring is formed at the interface due to the formation of a complex ion, which is `Fe(H2O)5NO^2+`. This complex is responsible for the brown color observed.

4. **Effect of Heating or Shaking**:
- If the mixture is heated or shaken, the following occurs:
- The NO (nitric oxide) gas is evolved from the reaction.
- The escape of NO gas prevents it from reacting with the `Fe^2+` ions to form the brown ring complex.
- As a result, the brown color disappears.

5. **Formation of Yellow Solution**:
- As the NO gas escapes, the remaining `Fe^2+` ions react with the concentrated sulfuric acid to form `Fe2(SO4)3`, which is a yellow solution. This indicates that the iron is now in the +3 oxidation state.

### Summary of Observations:
- (I) The brown color disappears due to the evolution of NO gas.
- (II) NO is indeed evolved during the reaction.
- (III) A yellow solution of `Fe2(SO4)3` is formed as a result of the oxidation of `Fe^2+` to `Fe^3+`.

### Conclusion:
All three statements (I, II, and III) are correct when the mixture is heated or shaken.