Terms for the Flammability of Materials

No single parameter defines flammability. Commonly used terms are flash point, flammability limits, autoignition temperature, minimum ignition energy and burning velocity. 

The flash point of a substance is often treated as the principal index of flammability, especially for liquids. The lower the flash point, the more flammable the substance. The flammability hazard of a substance is also increased by  

All substances in the form of liquids (and even many solids) possess a type of molecular motion that results in the escape of molecules from their surface in the form of vapor when they are not confined. When a liquid is left in an open container at room temperature, its molecules evaporate. When the liquid is confined in a partially full container that is dosed, the molecules will continue to escape from the surface however, because they cannot escape from the closed container, some of the molecules will return to liquid. Within a short time, an equilibrium will be achieved between the number of molecules escaping from the surface and those returning to the surface of the liquid. 

When this equilibrium occurs, a certain pressure will be exerted in the empty space above the liquid in the closed container. This is called the vapor pressure of the liquid. 

The boiling points of different types of liquids vary widely. They are an important physical characteristic both of liquids and of the many solids that melt to become liquids and then boil at a certain characteristic temperature. 

---