Final answer:
A vapor above its upper flammable limit is too rich to burn because the excess of fuel displaces oxygen, preventing combustion.
Step-by-step explanation:
A vapor that is above its upper flammable limit is said to be too rich to burn. The upper flammable limit (UFL) or upper explosive limit (UEL) refers to the maximum concentration of vapor in air above which ignition cannot occur. The mixture is too "rich" because it has too much fuel vapor and not enough oxygen to support combustion. For a combustion reaction to take place, there must be a certain proportion of fuel and oxygen; thus, when a vapor's concentration exceeds the UFL, the excess fuel displaces the oxygen necessary for combustion, making it incombustible.