🔍 Key Concepts

• Upper Explosive Limit (UEL) vs Lower Explosive Limit (LEL) • Meaning of a mixture being "too rich" or "too lean" to burn • What part of the mixture is being measured: vapor concentration vs oxygen concentration

💭 Think About

• Think about what happens when you keep adding more flammable vapor to air after you pass the ideal explosive range—does the mixture stop or start burning? • Which term (UEL or LEL) refers to the point where there is just barely enough vapor to ignite, and which refers to the point where there is so much vapor that it actually will not burn? • Look carefully at the answer choices: are they talking about the amount of vapor, the amount of oxygen, or the range where an explosion can occur?

✅ Before You Answer

• Make sure the definition you choose clearly refers to the upper limit of flammable vapor concentration, not oxygen amount. • Eliminate any option that actually describes the lower explosive limit (LEL) rather than the upper one. • Confirm that the option you choose correctly reflects a mixture that is too rich to burn beyond that limit.