Before adding the reagents of group III, the solution is heated with some concentrated `HNO_(3)` in order to :

Options

A. oxidise `Fe^(2+)` to `Fe^(3+)`
B. oxidise `Cr^(3+)` to `Cr_(2)O_(7)^(2-)`
C. lower than pH
D. increase the `NO_(3)^(-)`

Answer

To answer the question regarding why concentrated nitric acid (HNO₃) is added before the reagents of group III in qualitative inorganic analysis, we can break down the reasoning into several steps:

### Step-by-Step Solution:

1. **Understanding the Purpose of Heating with HNO₃**:
- The primary reason for heating the solution with concentrated nitric acid before adding the reagents of group III is to ensure that certain metal ions are in their correct oxidation states for identification.

2. **Oxidation of Ferrous to Ferric**:
- One of the key reactions that occur is the oxidation of ferrous ions (Fe²⁺) to ferric ions (Fe³⁺). This is crucial because group III cations include ferric ions, and ferrous ions would not precipitate correctly with the reagents used for group III.

3. **Oxidation of Chromium**:
- Additionally, concentrated nitric acid helps in oxidizing chromium from its lower oxidation state to the higher oxidation state (Cr³⁺ to Cr₂O₇²⁻). This ensures that chromium is present in a form that can be identified in the subsequent tests.

4. **Lowering pH**:
- The addition of concentrated HNO₃ also helps in lowering the pH of the solution. A lower pH is necessary for the precipitation reactions that will follow.

5. **Increasing Nitrate Ion Concentration**:
- Concentrated nitric acid increases the concentration of nitrate ions (NO₃⁻) in the solution, which can also influence the solubility of certain compounds and assist in the identification process.

6. **Conclusion**:
- Therefore, the overall purpose of adding concentrated nitric acid before proceeding with group III reagents is to ensure that all relevant metal ions are in their correct oxidation states and that the solution conditions are optimal for the qualitative analysis.

### Final Answer:
Before adding the reagents of group III, concentrated nitric acid is added to oxidize ferrous ions (Fe²⁺) to ferric ions (Fe³⁺), oxidize chromium, lower the pH, and increase the concentration of nitrate ions.