The highest concentration of a substance that will burn or explode when an ignition source is present is known as what?

The highest concentration of a substance that will burn or explode when an ignition source is present is known as what?

• A. Lower explosive limit
• B. Upper explosive limit
• C. Ignition temperature
• D. Flashpoint

Answer: (B)

Understanding the explosive limits of a vapor in air is what this item is about. The upper explosive limit is the highest concentration of vapor in air that can still ignite when there's an ignition source. Within the range between the lower explosive limit and the upper explosive limit, a mixture can ignite; below the lower limit the mixture is too lean to ignite, and above the upper limit it's too rich to ignite. The reason the upper limit is correct here is that it marks the boundary where there is just enough fuel to burn with the available oxygen; adding more fuel beyond that makes the mixture unable to sustain combustion. Ignition temperature and flashpoint describe energy or temperature thresholds rather than the effect of fuel concentration in air: ignition temperature tells you how hot something must be to ignite, while flashpoint is about the temperature at which vapors will ignite under test conditions, not about the concentration range in air.

Understanding the explosive limits of a vapor in air is what this item is about. The upper explosive limit is the highest concentration of vapor in air that can still ignite when there's an ignition source. Within the range between the lower explosive limit and the upper explosive limit, a mixture can ignite; below the lower limit the mixture is too lean to ignite, and above the upper limit it's too rich to ignite. The reason the upper limit is correct here is that it marks the boundary where there is just enough fuel to burn with the available oxygen; adding more fuel beyond that makes the mixture unable to sustain combustion. Ignition temperature and flashpoint describe energy or temperature thresholds rather than the effect of fuel concentration in air: ignition temperature tells you how hot something must be to ignite, while flashpoint is about the temperature at which vapors will ignite under test conditions, not about the concentration range in air.