Which statement defines the flash point?

• A. It is the temperature at which vapors ignite spontaneously in air
• B. It is the temperature at which vapors ignite and sustain a flame
• C. It is the temperature at which the liquid boils away
• D. It is the lowest temperature at which the liquid emits enough vapor to ignite briefly

Answer: (D)

Flash point is the lowest temperature at which a liquid can produce enough vapor to form an ignitable mixture with air and be capable of ignition if an ignition source is present. That momentary ignition, a brief flash, happens because the vapor concentration meets the threshold for ignition but not enough vapor to sustain a flame on its own. The other ideas refer to different concepts: spontaneous or autoignition without a flame, which occurs at a higher temperature; the fire point, where a flame can be sustained; and the boiling point, which is about the liquid turning into vapor rather than forming an ignitable mixture. So the statement describing the lowest temperature at which enough vapor is emitted to ignite briefly matches the definition of the flash point.