Colourless gas that has oxidising as well as reducing properties:

Options

A. `CO_(2)`
B. `SO_(2)`
C. `NO_(2)`
D. `SO_(3)`

Explanation

To determine the colorless gas that has both oxidizing and reducing properties, we can analyze the options provided step by step.

### Step-by-Step Solution:

1. **Identify the Options**: The options given are CO2, SO2, NO2, and SO3.

2. **Examine CO2**:
- **Color**: CO2 is a colorless gas.
- **Oxidation State**: The oxidation state of carbon in CO2 is +4.
- **Properties**: Since carbon is in its highest oxidation state (+4), CO2 cannot act as a reducing agent (it cannot be oxidized further). Therefore, CO2 does not have both oxidizing and reducing properties.

3. **Examine SO2**:
- **Color**: SO2 is also a colorless gas.
- **Oxidation State**: The oxidation state of sulfur in SO2 is +4.
- **Properties**: Sulfur in SO2 can be oxidized to +6 (as in SO4^2-) and can be reduced to 0 (elemental sulfur). This means SO2 can act as both an oxidizing agent and a reducing agent. Therefore, SO2 meets the criteria.

4. **Examine NO2**:
- **Color**: NO2 is not colorless; it is reddish-brown.
- **Conclusion**: Since it is not colorless, NO2 cannot be the answer.

5. **Examine SO3**:
- **Color**: SO3 is a colorless gas.
- **Oxidation State**: The oxidation state of sulfur in SO3 is +6.
- **Properties**: Since sulfur is already in its highest oxidation state (+6), SO3 cannot act as a reducing agent (it cannot be oxidized further). Thus, SO3 does not have both oxidizing and reducing properties.

6. **Final Conclusion**: The only gas that is colorless and has both oxidizing and reducing properties is SO2.

### Answer:
The colorless gas that has oxidizing as well as reducing properties is **SO2**.