Remember that scene toward the end of “The Bourne Identity” (a really good film) when Jason Bourne shoots the fuel tank in the backyard and it explodes? Yeah, no. Wouldn’t happen. Ditto the car that blows up after getting in a wreck or after the fuel tank is shot.
Somewhere, I know I’ve watch a scene in a movie where Character A douses Character B with gasoline and lights a Zippo, threatening B-boy with immolation if he doesn’t give up the wanted information. That won’t work either because they’d both be consumed by the same fireball.
Under tightly-controlled-don’t-try-this-at-home conditions, you can extinguish a match in a can of gasoline. This is because . . .
No liquids burn. And with the exception of some metallic substances, no solids burn either. Only gases and vapors burn.
Vapors are created as liquids evaporate (create vapor). They are the same chemical composition as the liquids from which they are derived, and if they are cooled, they will condense back into liquid form.
A gas is in a gaseous state at normal atmospheric pressure and temperature. When pressurized, gases will condense into liquids, but the instant the containment is breached, the liquid will convert instantly to a gas.
On the coldest day of the year in most parts of the world, if you put a match into a puddle of gasoline, you’ll get a fireball because the flash point of gasoline is about -50 degrees Fahrenheit. (“Flash point” has nothing to do with a visible flash of light. When a liquid evaporates [creates vapor], the technical term for that is to flash. The “flash point” is the temperature at which a liquid begins to create combustible vapors. Given the topic, it’s an unfortunate source of confusion.)
By comparison, the flashpoint of diesel fuel is between 125 and 180 degrees Fahrenheit. On that coldest day, you’d have a hard time getting diesel to ignite because there’d be no vapors to burn.
Back when my Big Boy Job had me teaching hazardous materials response classes to corporations, one of my best clients was a company that did hardhat diving into million-gallon tanks of flammable liquids like toluene to use cutting torches to fix plumbing deep inside the tank without emptying it. There was no chance of ignition because there are no vapors in the middle of a liquid. Along the surface of the tank, it gets a little dicey, though.
But The Sign Says “Flammable Liquid”
There’s not a lot of room for nuance or subtlety on a hazmat placard. The US Department of Transportation decided decades ago that first responders should know the difference between a milk truck and a gasoline truck. They came up with their Emergency Response Guidebook (ERG). By their definition, a “flammable liquid” is one that has a flashpoint below 100 degrees Fahrenheit. A “combustible liquid” is one with a flash point between 100 and 200 degrees Fahrenheit. Labels notwithstanding, liquids still don’t burn.
Vapors Displace Oxygen, and Nothing Burns Without Oxygen
When you fill the gas tank in your car, you don’t really fill it. You leave a vapor space in the top of the tank. Those vapors displace the ambient atmosphere inside the, bringing the oxygen levels down to nearly nothing.
In your story, when you shoot a car in its gas tank, the bullet tears through a lot of liquid and a lot of vapor, but since there’s no oxygen, there’ll be no explosion. More likely, the gasoline will leak out of the bullet hole. Once exposed to the atmosphere, the spilled gas will begin to evaporate and then the vapors can burn. As more liquid spills, the fire will get bigger, but it’s hard to conceive of the circumstance where you’d get a “bang” from the gasoline. A “whump” is more feasible.
Most Flammable Vapors Are Heavier Than Air
A lot heavier, in fact. When we create that puddle of gasoline, the vapors won’t rise. If we’re at elevation, they will flow down to the lowest point. If we’re on a flat surface, they will spread out, making the hazard area of the spill much, much larger.
Uncontained Liquids Will Evaporate
Let’s go back to the guy we doused in gasoline. All that liquid we poured on him is creating an invisible vapor cloud. If we’re close enough to talk, we’re enveloped in the same vapor cloud. When you thumb that Zippo, you’re likely to have as bad a day as your intended victim.
Does It Matter?
Here’s the question I struggle with when I address the real aspects of guns and hazmats: Does it matter? Should a film director care that the really cool scene couldn’t happen in real life, or should he just go with the really cool scene? After all, we write fiction.
What say you? Does it matter?