One gas bleaches the colour of flowers by reduction, while the other by oxidation, the two gases respectively are:

Options

A. `CO and Cl_2`
B. `H_2S and Br_2`
C. `NH_3 and SO_3`
D. `SO_2 and Cl_2`

Answer

To solve the question regarding the two gases that bleach the color of flowers by reduction and oxidation, we will analyze the properties and reactions of the gases involved.

### Step-by-Step Solution:

1. **Understanding Bleaching Agents**:
- Bleaching agents can work through two mechanisms: oxidation and reduction.
- Oxidizing agents remove electrons from the colored compounds, leading to decolorization, while reducing agents donate electrons, also resulting in loss of color.

2. **Identifying the Gases**:
- The question specifies two gases that bleach flowers through these mechanisms.
- Common gases known for their bleaching properties include chlorine (Cl₂) and sulfur dioxide (SO₂).

3. **Chlorine Gas (Cl₂)**:
- Chlorine acts as an oxidizing agent.
- When chlorine gas reacts with water, it produces nascent oxygen (O), which can oxidize colored substances, leading to bleaching.
- The reaction can be represented as:
\[ \text{Cl}_2 + \text{H}_2\text{O} \rightarrow 2 \text{HCl} + \text{O (nascent)} \]

4. **Sulfur Dioxide (SO₂)**:
- Sulfur dioxide acts as a reducing agent.
- It can react with colored substances to release oxygen from them, resulting in the loss of color.
- The reaction can be represented as:
\[ \text{SO}_2 + 2 \text{H}_2 \rightarrow \text{H}_2\text{SO}_4 + 2 \text{H} \]
- Here, the hydrogen (H) acts as a reducing agent.

5. **Conclusion**:
- The gas that bleaches by reduction is sulfur dioxide (SO₂), and the gas that bleaches by oxidation is chlorine (Cl₂).
- Therefore, the two gases respectively are sulfur dioxide and chlorine gas.

### Final Answer:
The two gases that bleach the color of flowers by reduction and oxidation are **sulfur dioxide (SO₂)** and **chlorine gas (Cl₂)**.