Even in the absence of group II,colloidal yellowish ppt appears on passing `H_(2)S` gas , it may be due to the presence of (a) _____.

Answer

To solve the question regarding the appearance of a colloidal yellowish precipitate upon passing H₂S gas, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding the Context**: The question states that a colloidal yellowish precipitate appears when H₂S gas is passed, even in the absence of group II elements. We need to identify what could cause this phenomenon.

2. **Identifying the Role of H₂S**: When H₂S gas is introduced, it dissociates to form sulfide ions (S²⁻). Normally, these sulfide ions can react with certain metal ions to form precipitates.

3. **Considering the Absence of Group II Elements**: Since the question specifies the absence of group II elements, we need to consider other possible sources that could lead to the formation of a yellowish precipitate.

4. **Identifying Possible Oxidizing Agents**: The video transcript mentions that oxidizing agents such as sulfite ions (SO₃²⁻) or nitrate ions (NO₃⁻) can be present. These agents can oxidize the sulfide ions present in H₂S.

5. **Understanding the Reaction**: The oxidizing agents (SO₃²⁻ or NO₃⁻) can oxidize the sulfide ions (S²⁻) to form free sulfur (S). This free sulfur can then form a colloidal yellowish precipitate.

6. **Conclusion**: Therefore, the yellowish precipitate observed is due to the presence of oxidizing agents like SO₃²⁻ or NO₃⁻, which convert sulfide ions into free sulfur.

### Final Answer:
The blank can be filled with: **Oxidizing agents like SO₃²⁻ or NO₃⁻.**

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