http://ehs.unc.edu/manuals/laboratory/10-2.shtmlFlammable liquids have a flash point below 100°F (37.7°C) and a vapor pressure not exceeding 40 psi (276 kPa). By contrast, combustible liquids have a flash point at, or above, 100°F (37.7°C). Classes of flammable and combustible liquids are further defined in Appendix 10-A.
The flash point is the lowest temperature, as determined by standard tests, at which a liquid gives off vapor in sufficient concentration to form an ignitable mixture with air near the surface of the liquid within the test vessel. Many common laboratory solvents and chemicals have flash points that are lower than room temperature.
Actually, the vapor, not the liquid, burns. The rate at which different liquids produce flammable vapors depends on their vapor pressure. The degree of fire hazard depends also on the ability to form combustible or explosive mixtures with air.
