Which of the following statements is/are correct?

Options

A. In lassaigne's test for halogens, conc. `HNO_3` is used to remove `HCN` and `H_2S`
B. When an organic compound is heated with dry `CuO` and the gases evolved are passed through lime water which turns milky, the gass may be `CO_2` or `SO_2`.
C. In Carius method, sulphur is oxidised to `SO_4^(2-)` ion with fuming `HNO_3`
D. In lassaigne's test, N present in the organic compound is converted into `CN^(-)` ions.

Explanation

To determine which statements are correct regarding the purification of organic compounds and qualitative analysis, we will analyze each statement one by one.

### Step-by-Step Solution:

1. **Statement A**: In the Lassaigne test for halogens, concentrated nitric acid is used to remove HCN and H2S.
- **Analysis**: This statement is correct. In the Lassaigne test, the organic compound is first boiled with concentrated nitric acid (HNO3) to convert cyanide ions (CN-) into hydrogen cyanide (HCN) and sulfide ions (S2-) into hydrogen sulfide (H2S). This step is crucial as these radicals can interfere with the detection of halogens.
- **Conclusion**: Statement A is correct.

2. **Statement B**: When an organic compound is heated with CuO and the gas evolved is passed through lime water which turns milky, the gas may be CO2 or SO2.
- **Analysis**: This statement is also correct. When organic compounds containing carbon and sulfur are heated with copper oxide (CuO), they can produce carbon dioxide (CO2) and sulfur dioxide (SO2). Both gases will react with lime water (calcium hydroxide) to form calcium carbonate (CaCO3) and calcium sulfide (CaS), respectively, causing the lime water to turn milky.
- **Conclusion**: Statement B is correct.

3. **Statement C**: In the KDS method, sulfur is oxidized to sulfate ion with fuming HNO3.
- **Analysis**: This statement is correct. In the KDS (Kjeldahl's) method, sulfur can be oxidized to sulfate ions (SO4^2-) when treated with fuming nitric acid (HNO3). The resulting sulfate can then be precipitated as barium sulfate (BaSO4) when treated with barium chloride (BaCl2).
- **Conclusion**: Statement C is correct.

4. **Statement D**: In the Lassaigne test for nitrogen, nitrogen present in the organic compound is converted into CN- ions.
- **Analysis**: This statement is correct. In the Lassaigne test for nitrogen, the organic compound containing nitrogen is fused with sodium metal, leading to the formation of sodium cyanide (NaCN), which contains CN- ions. This step is essential for detecting nitrogen in the organic compound.
- **Conclusion**: Statement D is correct.

### Final Conclusion:
All statements A, B, C, and D are correct.