At what point do vapors given off from a material form an ignitable mixture?

• A. the lower flammability limit
• B. the upper flammability limit
• C. the flash point
• D. the autoignition temperature

Answer: (C)

The key idea is the flash point—the temperature at which a liquid begins to emit enough vapor to mix with air and form an ignitable mixture near the surface. Once that vapor-air mix is present, an ignition source can cause a flame. The lower and upper flammability limits describe the range of vapor concentrations in air that can burn, not the temperature at which vapors start to form an ignitable mixture. The autoignition temperature is the temperature at which the material will ignite without any flame or spark, which is a different concept. So the flash point is the point where vapors become capable of forming an ignitable mixture with air.