Which gas is essential constituent of almost all fuel gases?

Explanation

To determine which gas is an essential constituent of almost all fuel gases, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Fuel Gases**:
- Fuel gases are those gases that can be used as fuel under normal conditions. They include various hydrocarbons and other gases.

2. **Identifying Common Components**:
- Fuel gases are primarily composed of hydrocarbons. Common hydrocarbons found in fuel gases include:
- Methane (CH₄)
- Propane (C₃H₈)
- Butane (C₄H₁₀)
- Additionally, other gases such as hydrogen (H₂) and carbon monoxide (CO) can also be present.

3. **Function of Fuel Gases**:
- These gases serve as sources of heat energy and can be easily transported through pipelines to where they are needed for combustion.

4. **Determining the Essential Gas**:
- Among the gases mentioned, carbon monoxide (CO) is a significant component that is found in almost all fuel gases. It is produced during the incomplete combustion of carbon-containing fuels.

5. **Conclusion**:
- Therefore, the gas that is an essential constituent of almost all fuel gases is **carbon monoxide (CO)**.

### Final Answer:
The essential constituent of almost all fuel gases is **carbon monoxide (CO)**.

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