Which term identifies the temperature at which a liquid will emit enough vapor to form a flammable mixture with air and be ignitable?

• A. Flash point.
• B. Auto ignition temperature.
• C. LEL.
• D. UEL.

Answer: (A)

Flash point is the lowest temperature at which a liquid will emit enough vapor to form an ignitable mixture with air. Below this temperature, there isn’t enough vapor present to reach the flammable concentration, so ignition isn’t possible even if an ignition source is nearby. When the liquid reaches the flash point, the vapor above it can ignite if there’s a flame or spark. This is different from the auto-ignition temperature, which is the temperature at which the vapor will ignite on its own without any flame or spark. The LEL and UEL describe the range of vapor concentrations that can ignite in air, not the temperature at which vapors begin to form, so they don’t answer the question.