Propane is preferred in hot air balloons over butane because it has a lower

• A. boiling point
• B. vapor pressure
• C. molecular weight
• D. flammability

Answer: (A)

Fuel vaporization at operating temperatures determines how reliably the burner will produce a steady flame. Propane has a much lower boiling point than butane, about -42°C compared with around -0.5°C. Because of this, propane tends to stay in the gaseous state and vaporize readily even when conditions are cool or at altitude. In a hot air balloon burner, you need the fuel to be delivered as a gas to mix with air and burn smoothly; if the fuel can linger as a liquid, the flow becomes inconsistent and the flame can waver.

Butane’s higher boiling point means it risks remaining liquid or only partially vaporizing under colder conditions, which can disrupt the burn. So the lower boiling point of propane is what makes it more reliable for balloon operation across the range of temperatures you’ll encounter. The other factors—vapor pressure, molecular weight, and flammability—are less decisive for this specific reliability in vaporization.