When most substances burn, the actual combustion takes place after the liquid fuel has been vaporized.

When most substances burn, the actual combustion takes place after the liquid fuel has been vaporized.

• A. Vaporized
• B. Heated
• C. Ignited
• D. Cooled

Answer: (A)

Most fuels burn in the vapor phase—the fuel must become a vapor before combustion can occur. The liquid needs to vaporize so its molecules can mix with oxygen in the air and react in the flame. The energy that drives this transition is the heat of vaporization; once enough fuel is in the vapor form, ignition can occur and the flame is sustained as the vapor continues to oxidize. Simply heating the liquid without producing enough vapor won’t create a flame, and cooling would prevent vapor formation and hinder burning.

Most fuels burn in the vapor phase—the fuel must become a vapor before combustion can occur. The liquid needs to vaporize so its molecules can mix with oxygen in the air and react in the flame. The energy that drives this transition is the heat of vaporization; once enough fuel is in the vapor form, ignition can occur and the flame is sustained as the vapor continues to oxidize. Simply heating the liquid without producing enough vapor won’t create a flame, and cooling would prevent vapor formation and hinder burning.