Name any one harmful substance emitted when fossil fuels are burnt.

Answer

### Step-by-Step Text Solution:

1. **Understanding the Question**: The question asks for a harmful substance that is emitted when fossil fuels are burned.

2. **Identifying the Process**: When fossil fuels (such as coal, oil, and natural gas) are burned, they undergo a chemical reaction known as combustion.

3. **Recognizing Emissions**: During combustion, various substances are released into the atmosphere. One of the most significant emissions from burning fossil fuels is carbon dioxide (CO2).

4. **Explaining the Harmful Effects**: Carbon dioxide is considered harmful because it contributes to global warming. It traps heat in the Earth's atmosphere, leading to an increase in temperature and climate change.

5. **Conclusion**: Therefore, one harmful substance emitted when fossil fuels are burned is carbon dioxide (CO2).

### Final Answer:
One harmful substance emitted when fossil fuels are burnt is carbon dioxide (CO2).

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