For combustion to occur, what state must the fuel be in?

Combustion is a chemical reaction that typically requires a fuel to be in a vapor state, as this allows for a more effective interaction with the oxidizer (usually oxygen in the air). When fuel is vaporized, the increased surface area significantly enhances the rate at which the fuel can react with oxygen. This is especially important for hydrocarbon fuels, where the vapor phase allows for quick and efficient burning.

In the context of combustion, the vapor state facilitates the mixing of fuel and oxidizer, ensuring a homogeneous mixture that can ignite and sustain a flame. Most fuels, like gasoline or natural gas, are liquid at room temperature but must vaporize before they can combust. Thus, the requirement for combustion to occur reflects the necessity of fuel being in a vapor state to support the rapid reactions that define burning.

The other options are less conducive to combustion. Fuel does not need to be in a liquid state—while some types of combustion can occur with liquids, vaporization is essential for optimal combustion efficiency. Mixing fuel with carbon dioxide actually inhibits combustion, as carbon dioxide is a product of combustion and does not support the reaction. Lastly, operating below atmospheric pressure could disrupt the necessary conditions for sufficient oxygen levels, also rendering combustion inefficient. Therefore, the correct state