by Michael Capraro

Created on: January 18, 2008   Last Updated: January 20, 2008

Flash point is the lowest temperature at which a flammable liquid evolves sufficient vapor to form an ignitable mixture of those vapors in air.

Well, that's about it. Short article. So, what's it to you?

Consider an example you already (hopefully) know - you should never store or dispense gasoline in your basement. While regular unleaded gasoline contains chemicals like benzene, a human carcinogen, let's ignore the toxicity hazards. The more immediate hazard (acute effect) is blowing yourself up before you can develop cancer (chronic effect).

The flash point of regular unleaded gasoline is about minus 49 degrees F. It's a pretty safe bet your basement is warmer than that. So, if you handle gasoline in your basement, you will have a flammable vapor cloud of some magnitude - maybe a big one if you spill it on the floor. Flammable vapor clouds are very good at finding ignition sources.

OK, you don't smoke, have all explosion proof (XP) outlets and lighting in your basement (yeah, right), and the furnace is off for the summer - besides, it's clear across the basement from where you're working. Uh-huh, and do you have a gas water heater? It has a pilot flame. Gasoline vapors are 3 to 4 times more dense than air, so they tend to travel (you thought they were going to stay put?) along the floor. Guess where that pilot flame is located. You probably have drains in your basement, or else a sump. Liquid gasoline floats on and is insoluble in water. So regardless where the spill occurred, you can have vapors above all your drains, including that one right by the water heater. In the case of the sump, there's a sump pump. Did you remember to get the XP motor for that too? Sure you did.

Oh, you only spilled a little bit, a quart maybe. How big a vapor cloud could that form? Well, a quart of gasoline weighs close to 1.5 pounds (apologies to more enlightened readers, but I'll stick with English units). Gasoline is actually a mixture of organic compounds, but its average molecular weight (you remember - High School Chemistry) is about 108. So you spilled roughly 1.5/108 = 0.0135 pound moles. Doesn't sound like much. Well, one pound mole of liquid can form up to 359 cubic feet of vapor at STP, standard temperature & pressure as you learned it in High School. Your basement should be a bit warmer than that, so the number would be a bit bigger.

So, 0.0135 pound moles could form a small vapor cloud of almost five (4.85) cubic feet - but wait! That's 100% gasoline vapor. The vapors

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