What must accompany fuel vaporization?

• A. an absorption of heat
• B. a decrease in vapor pressure
• C. a reduction in volume
• D. an increase in density

Answer: (A)

Fuel vaporization is a phase change from liquid to gas, and it requires energy to overcome the intermolecular forces holding the liquid together. This energy input is the latent heat of vaporization, so heat must be absorbed from the surroundings as the liquid becomes vapor. In an engine, heat from the incoming air and engine surfaces provides that energy, enabling the fuel to vaporize so it can mix with air and burn efficiently. Vaporization does not involve a decrease in vapor pressure (in fact, adding heat increases vapor pressure and drives vaporization), and it does not align with a volume reduction or a density increase—gas has lower density and occupies more volume than liquid.