When a flammable liquid (such as gasoline or charcoal lighter fluid) ignites, it is not the liquid itself that burns: what actually happens is that the liquid vaporizes and the resulting air-vapor mixture burns If the temperature is such that the percentage of the vapor in the mixture is below a certain level (the lower flammable limit), the liquid will not ignite if exposed to a spark or other ignition source. A match may burn in the mixture, but the flame will not spread.

(a) The flash point of a liquid is the lowest temperature at which the liquid vaporizes sufficiently to form an ignitable mixture with air. For example, the flash point of octane at 1 atm is 13°C (55°F), which means that dropping a match into an open container of octane is likely to start a fire on a warm summer day but not on a cold winter day. (Please do not try it!) Suppose you are keeping two solvents in your laboratory--one with a flash point of 15°C and the other with a flash point of 75° C. How do these solvents differ from the standpoint of safety? How might you treat them differently?

(b) The lower flammable limit (LFL) of methanol in air is 6.0 mole%. Calculate the temperature at which the equilibrium percentage of methanol vapor in a saturated methanol-air mixture would equal the LFL. (This temperature is a rough estimate of the flash point.)

(c) Suppose an open container of methanol is kept at a temperature below the temperature calculated in part (b). Why would it still be unsafe to expose the container to a flame?