The very useful information profile on n-hexane on page 587 of the May 2001 issue of the Journal would benefit from inclusion of the flash point of this substance. This is especially so in view of the fact that the information is largely directed at students and teachers. If there is a spillage of an organic chemical in the teaching laboratory, emergency responses are all the more urgent if the chemical is above its flash point. However, n-hexane provides an example of the anomaly that a liquid can be too far above its flash point to form an ignitable mixture.

Authoritative sources including the SFPE Handbook give the closed-cup flash point of n-hexane as 251 K (-22 °C); therefore one would certainly, on those grounds, expect the compound to ignite when spilt at room temperature if the vapor is contacted by a flame. Roughly speaking, the flash point of any liquid is the temperature at which the equilibrium vapor pressure is such that in a mixture with air, total pressure one atmosphere, the proportion of the vapor corresponds to the lower flammability limit. Now n-hexane burns in air according to

We are told in the information profile in the Journal that the vapor pressure of n-hexane at 120 °C is 124 torr, so at this temperature, with a total pressure of one atmosphere, a mixture of air and n-hexane at its equilibrium vapor pressure would have a percentage

(124/760) x 100 = 16.3% (molar or volume basis)

of n-hexane. This is well in excess of the upper flammability limit of 7.4% (SFPE Handbook), so on those grounds the n-hexane would not ignite if spilt at room temperature and contacted by a flame. Of course, the assumption of equilibrium between the two phases of the hydrocarbon compound might not be justified following accidental spillage, so I certainly do not recommend reliance on the conclusion that the n-hexane/air mixture is too rich to ignite. Nevertheless, this does illustrate another facet to the understanding and application of flash points: that a liquid can in principle be too far above its flash point to form a flammable mixture with air.