Which of the following can produce a colour change from yellowish brown to light green in acidified aqueous solution of iron (III) chloride?

Options

A. `H_(2)` gas
B. `O_(2)`
C. Addition of zinc and HCI
D. Simple addition of HCI

Answer

To solve the question of which substance can produce a color change from yellowish brown to light green in an acidified aqueous solution of iron (III) chloride (FeCl3), we need to understand the chemistry involved in this color change.

### Step-by-Step Solution:

1. **Identify the Colors of Iron Compounds**:
- Iron (III) chloride (FeCl3) is yellowish brown in color.
- Iron (II) chloride (FeCl2) is light green in color.

2. **Understand the Reaction**:
- The color change from yellowish brown to light green indicates a reduction reaction where Fe(III) is reduced to Fe(II).
- This means that FeCl3 (Fe in +3 oxidation state) must be converted to FeCl2 (Fe in +2 oxidation state).

3. **Determine the Reducing Agent**:
- To achieve this reduction, we need a reducing agent that can provide nascent hydrogen (H) to reduce Fe(III) to Fe(II).
- Nascent hydrogen can be produced from certain reactions, such as the reaction of zinc with hydrochloric acid (HCl).

4. **Evaluate the Options**:
- **Option 1**: H2 gas alone does not provide nascent hydrogen.
- **Option 2**: O2 cannot act as a reducing agent.
- **Option 3**: Zinc (Zn) with HCl produces zinc chloride (ZnCl2) and nascent hydrogen, which can reduce FeCl3 to FeCl2.
- **Option 4**: Simple addition of HCl does not produce nascent hydrogen.

5. **Conclusion**:
- The only option that can produce the necessary nascent hydrogen to reduce FeCl3 to FeCl2, resulting in a color change from yellowish brown to light green, is the addition of zinc and hydrochloric acid (Option 3).

### Final Answer:
The correct option is **the addition of Zinc and HCl**.