The upper and lower vapor concentrations that will produce a flame at a given pressure and temperature are called what?

• A. Flammable and explosive limits
• B. Autoignition thresholds
• C. Boiling point ranges
• D. Vapor pressure limits

Answer: (A)

The concept being tested is the range of vapor concentrations in air that will support flame propagation at a given temperature and pressure. This window is called the flammable limits, also known as the explosive limits. Within this range, there is a lower limit—the minimum concentration needed for the flame to sustain itself—and an upper limit—the maximum concentration at which the flame can still propagate. Below the lower limit the mixture is too lean to burn; above the upper limit the mixture is too rich to burn. These limits depend on temperature, pressure, and the specific fuel, and they guide safety decisions such as ventilation, storage, and ignition risk assessments.

Autoignition thresholds refer to the temperature at which a material will ignite without an external flame. Boiling point ranges relate to phase changes, not flame propagation. Vapor pressure limits are not a standard term for this concept.