In which state must a fuel be in for combustion to take place?

For combustion to occur, fuel must be in a state that allows it to react with an oxidizer, typically oxygen, at elevated temperatures. In this context, the vapor state is crucial because it provides the most effective mixture of fuel and air, facilitating ignitability.

When fuel is in vapor form, it is composed of tiny particles that can easily mix with atmospheric oxygen. This mixture enables the chemical reaction required for combustion. In practice, many fuels exist as liquids or solids but must be converted into vapors to ignite and sustain a flame. For example, gasoline (a liquid) needs to evaporate into vapor to burn effectively. Although solids and liquids can contribute to combustion processes, it is the vapor phase that plays the most significant role, as it is during this phase that the fuel can mix efficiently with air for the combustion reaction to initiate.

The other states, while potentially contributing to combustion as part of a broader fuel mixture, do not represent the ideal conditions for combustion initiation.