Which of the following statements is/are incorrect ?

Options

A. In thiourea test for nitrite , a green coloured solution is obtained
B. It is not necessary to carry out the chromyl chloride test in a dry test tube
C. In `PbNO_3` , the brown ring test can be performed with its water extract
D. Suspension of `CdCO_3` gives black precipitate with sodium sulphide solution

Explanation

To determine which statements are incorrect, we will analyze each statement one by one based on the information provided in the video transcript.

### Step-by-Step Solution:

1. **Statement Analysis:**
- **Statement 1:** "In thiourea test for nitrite, a green colored solution is obtained."
- This statement is **incorrect**. The thiourea test for nitrite actually produces a **deep red coloration** due to the formation of Fe(SCN)₃ (iron(III) thiocyanate).

2. **Statement 2:** "It is not necessary to carry out the chromyl chloride test in a dry test tube."
- This statement is **incorrect**. The chromyl chloride test must be performed in a **dry test tube** because if moisture is present, chromyl chloride (CrO₂Cl₂) reacts with water to form H₂CrO₄ and HCl, which would interfere with the test.

3. **Statement 3:** "In Pb(NO₃)₂, the brown ring test can be performed with its water extract."
- This statement is **incorrect**. The brown ring test cannot be performed with lead nitrate's water extract because the reaction produces PbSO₄, which is a white precipitate, thus the brown ring will not be visible.

4. **Statement 4:** "Suspension of CdCO₃ gives black precipitate with sodium sulfide solution."
- This statement is **incorrect**. When CdCO₃ is treated with sodium sulfide, it produces a **yellow precipitate** of cadmium sulfide (CdS), not a black precipitate.

### Conclusion:
All the statements provided in the question are incorrect. Therefore, the answer is that options A, B, C, and D are all incorrect.