In hot air balloons, propane is preferred to butane or other hydrocarbons because it

• A. has a lower boiling point
• B. is cheaper
• C. is heavier than air
• D. is less flammable

Answer: (A)

Propane is preferred because its lower boiling point means it vaporizes to a gas readily across a wide range of operating temperatures, especially the cooler conditions encountered at altitude. In a hot air balloon burner you need the fuel to be in the gaseous form to mix with air and burn smoothly, so a fuel that stays readily vaporized provides a steady, controllable flame. Butane has a higher boiling point, so it can be harder to vaporize in cold conditions, leading to less reliable flame and control. The other reasons listed don’t address how the burner actually maintains a stable, controllable flame, which is driven by how easily the fuel vaporizes.