Alkanes burn with a smoky flame.

Answer

**Step-by-Step Solution:**

1. **Understanding Alkanes**: Alkanes are saturated hydrocarbons, which means they contain only single bonds between carbon atoms. Their general formula is CnH2n+2.

2. **Combustion of Alkanes**: When alkanes undergo combustion, they react with oxygen (O2) to produce carbon dioxide (CO2) and water (H2O). The reaction can be represented as:
\[ \text{C}_n\text{H}_{2n+2} + (3n + 1)\text{O}_2 \rightarrow n\text{CO}_2 + (n + 1)\text{H}_2\text{O} \]

3. **Complete vs. Incomplete Combustion**: Complete combustion occurs when there is a sufficient supply of oxygen, resulting in the production of carbon dioxide and water. Incomplete combustion occurs when there is insufficient oxygen, which can lead to the formation of carbon monoxide (CO) and soot (carbon particles), causing a smoky flame.

4. **Smoky Flame**: The statement that "alkanes burn with a smoky flame" is incorrect. Alkanes typically burn with a clean flame because they undergo complete combustion when enough oxygen is present. The smoky flame is more characteristic of hydrocarbons that are unsaturated (like alkenes and alkynes) or when there is a limited supply of oxygen.

5. **Conclusion**: Therefore, the correct assertion is that alkanes do not burn with a smoky flame; they burn with a clean flame under complete combustion conditions.