A flash point test is usually performed on volatile organic liquids. Open cup flash point and closed cup flash point are the most common tests. The flash point of a liquid is the temperature at which the vapors can be ignited by an ignition source. Volatile liquids have a vapor pressure which is dependent upon temperature. The concentration of the vapor of a volatile liquid in the air above the surface of the liquid increases with increasing temperature. The temperature at which this mixture of vapor and air can be ignited with an ignition source is the flash point of the liquid. Flammable liquids are defined as liquids which have a flash point below 100°F when tested by closed-cup test methods. A combustible liquid is one whose flash point is 100°F or higher, also when tested by closed-cup methods, according to the National Fire Protection Association (NFPA).
Flash point is not the same as autoignition temperature. The autoignition temperature is the temperature at which a chemical will ignite without an ignition source such as a spark or flame. In an internal combustion engine ignited by a spark the fuel should have a low flash point but a high autoignition temperature like gasoline. In an internal combustion engine which uses compression the fuel should have a high flash point and a low auto ignition temperature like diesel fuel.