Explosive 5/1/21

Science with Richard Bleil

If you take a barrel of gasoline and light it on fire, you’ll get a barrel of burning gasoline. If you let that same gasoline vaporize to a high enough concentration and put a match to it, it will explode.

Have you ever wondered what the difference is? Not long ago, I posted a tongue-in-cheek blog about what it would be like to live in a movie. A friend of mine pointed out something I had forgotten. After a crash, the cars always explode. Always. But have you ever actually seen a car explode after a crash? Depending on how schlocky (I kind of like that word) the movie, the car bursts into flames just by driving over the cliff even before it hits anything.

In a fire or in an explosion, the total chemical potential energy is actually the same. The only real difference is how rapidly that energy is converted into heat or shock waves. In a vapor explosion, the gas molecules are mixed in the air with oxygen (the only component of air that actually is responsible for combustion). Because of the way it’s mixed, once the reaction begins it can propagate immediately throughout the mixture releasing the energy exponentially. In a barrel of gasoline, on the other hand, the fuel can only react where oxygen is present, but that oxygen is only at the surface. This means that as the surface gasoline burns (don’t try this), the rest of the gasoline remains liquid and that chemical potential remains stored.

It’s all in the oxygen.

What happens if you don’t want to vaporize the gasoline but still want an explosion? This is what happens when terrorists like Timothy McVeigh wanted when he blew up a truck in front of a federal building in Oklahoma City in 1995. It doesn’t do a lot of good to bubble oxygen through the gasoline because the bubbles will just surface again. Besides, even before they do, the surface area in contact with the gasoline in the barrel is fairly small compared to the total volume of the gasoline. However, if you mix an oxidizing agent with the gasoline, that’s kind of like dispersing gasoline and oxygen the same way that it can in the vapor form. In the burning process, the oxygen basically is taking electrons away from the fuel (this is the chemical definition of “oxidation”) in a violent and highly energetic process. An oxidizing agent does the same thing, so it’s as if the burning process is occurring throughout the barrel all at the same time, releasing all of that energy rapidly.

The biggest difference between an explosion and the burning process is the shock wave. In the burning of fuel, it’s not just heat that is released (and a lot of it), but also gasses, primarily carbon dioxide and water vapor. When this happens in burning, the gases and heat are dispersed in a relatively slow process of diffusion (flow of gasses) and convection (flow of heat). But in an explosion, all of that heat, and all of those gasses, are suddenly created and exist in a volume far too small for stability. Heat is kinetic energy, so the higher the heat the faster molecules move. Suddenly, there is excessive pressure in a localized volume by the sudden creation of new gasses and incredible temperatures. These gasses are so energetic and repel each other because of their proximity so much that they rush to get away from each other with great force. This is the shock wave. It’s the shock wave, not the fire, that caused the federal building in Oklahoma City to take so much damage. In fact, as I recall, there were not even any fires in the building even though the entire face of it, and about half of the building was in rubble.

It is possible to create these shock waves even without the heat. We’ve seen explosions of this nature before. If you drop a bottle of a carbonated beverage, the internal pressure can build up so rapidly that it literally blows the bottle apart. This is such an explosion. As water freezes, it expands which can cause increased pressure and explosions along a household waterline blowing out the plumbing.

I like shooting guns. I only shoot at targets as I don’t think it’s in me to actually take a life, animal or otherwise, but this is an explosion as well. In the bullet, there is an explosive in the casing. The firing pin hits basically a tiny blasting cap at the base of the bullet creating the spark necessary to ignite the gunpowder. It basically burns rapidly, creating gasses and heat which is contained in the metal casing, but that metal casing has a weak point where it holds the slug. The casing is strong enough to allow, just for an instant, that heat and those gasses to build up excessive pressure until that weak point gives out releasing the slug. As it releases, the resulting explosion has directionality, pushing the slug out through the barrel of the gun. Of course, Newton tells us that as it pushes forwards, it also pushes backwards. This is the recoil of the gun, and in a semi-automatic gun it also pushes the slide of the gun backwards at the same time as the slug is pushed forward allowing for the ejection of the now empty casing and insertion of the next round into the barrel.


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