Vegetable colouring matter in presence of moisture is bleached by `SO_2` due to ___.

Answer

To solve the question "Vegetable colouring matter in presence of moisture is bleached by SO2 due to ___," we can follow these steps:

### Step 1: Understand the Role of SO2
Sulfur dioxide (SO2) is known for its properties as a bleaching agent. It interacts with various substances, including vegetable coloring matter, especially in the presence of moisture.

**Hint:** Think about the chemical properties of SO2 and how it interacts with organic compounds.

### Step 2: Identify the Mechanism of Bleaching
The bleaching action of SO2 is primarily due to its ability to act as a reducing agent. This means that it can donate electrons to other substances, leading to a reduction reaction.

**Hint:** Recall the definitions of oxidizing and reducing agents in chemistry.

### Step 3: Reaction with Moisture
When SO2 is present with moisture (water), it can form sulfurous acid (H2SO3) or sulfuric acid (H2SO4). This acidic environment can facilitate the reduction of colored pigments found in vegetable matter.

**Hint:** Consider how acids can influence chemical reactions, especially in the context of color changes.

### Step 4: Formation of Colorless Compounds
The colored pigments in vegetables are reduced to colorless compounds when they react with the reducing agent (SO2). This is the essence of the bleaching process.

**Hint:** Think about what happens to pigments when they undergo reduction.

### Step 5: Conclusion
Thus, the final answer to the question is that vegetable coloring matter in the presence of moisture is bleached by SO2 due to the reducing action of sulfur dioxide.

**Final Answer:** The bleaching is due to the reducing action of sulfur dioxide.

### Summary of Steps:
1. Understand the role of SO2 as a bleaching agent.
2. Identify its mechanism as a reducing agent.
3. Consider the reaction with moisture.
4. Recognize the formation of colorless compounds.
5. Conclude with the reason for the bleaching action.