Which of the following gases has the blue flame?

Options

A. CO
B. `O_2`
C. `N_2`
D. `CO_2`

Explanation

To determine which of the gases has a blue flame, we need to understand the concept of combustion and what a blue flame signifies.

### Step-by-Step Solution:

1. **Understanding Combustion**:
- Combustion is a chemical reaction that occurs when a substance reacts with oxygen, releasing energy in the form of heat and light.

2. **Types of Flames**:
- Flames can vary in color depending on the completeness of the combustion. A blue flame indicates complete combustion, where the fuel burns efficiently with sufficient oxygen.

3. **Identifying the Gases**:
- The gases provided in the options are carbon monoxide (CO), oxygen (O2), nitrogen (N2), and carbon dioxide (CO2).

4. **Analyzing Each Gas**:
- **Carbon Monoxide (CO)**: Burns with a blue flame when it undergoes complete combustion in the presence of oxygen, producing carbon dioxide (CO2).
- **Oxygen (O2)**: Supports combustion but does not burn itself; it does not produce a flame.
- **Nitrogen (N2)**: Inert gas that does not support combustion; it does not produce a flame.
- **Carbon Dioxide (CO2)**: A product of combustion and does not burn; it does not produce a flame.

5. **Conclusion**:
- Among the given options, carbon monoxide (CO) is the gas that burns with a blue flame, indicating complete combustion.

### Final Answer:
The gas that has a blue flame is **carbon monoxide (CO)**.

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