Friday, June 5, 2009

on the importance of being earnest about oxygen

Last year in lab, this guy got burnt semi-badly when his oil bath caught fire (actually this story has the odd quirk that I never saw the guy, so i don't know how badly he was hurt, but I can promise you he exists). Actually oil bath fires are not so dangerous, but how you extinguish them can be really dangerous. The molecule that explains why this is so is oxygen. Before I explain, watch this video to see how bad oil fires can get:

But before we talk about oxygen we're going to talk about water. In that video they throw water on the burning oil. What happens when the water hits the oil? burning oil is hot as hell dummy!! It boils!!!

When I worked at a Hardee's restaurant for a year or so in high school, we would use this metal canister to move some hot fryer oil from the fryer to the grill to help clean the grill off. We would always wash the metal canister before doing this, so it would be a little bit wet. I thought it was really cool when you would put the canister in the oil, the water would boil, and make neat bubbles. One day I decided to leave a little extra water in the canister. As I dipped the canister down, the oil flowed in onto the water, turning it all into a gas. Since gas molecules take up waaaaay more room than water molecules, and there was enough boiling water in there, it essentially created an oil cannon. It shot oil all over the ceiling, and shot my arm down into the fryer (luckily I was wearing a thick protective glove).

So in this case, an explosion of oil happened because the water started out on the bottom and forced the oil out of the canister. If you dump water onto burning oil, since water is more dense than oil, the water races to the bottom as it vaporizes, creating the same kind of cannon scenario.

Now in the oil fire scenario, when the oil is shot up into the air, it has more hot oil surfaces exposed to the air than it did before water was dumped on it. This gives that hot oil what it really wants... OXYGEN!! (remember the carbon cycle??) So instead of just the surface of the oil being on fire as before, because that was the only place the oil could get oxygen, now ALL THE OIL IS ON FIRE.

Returning to the lab accident I started with. Once this guy noticed his oil bath was on fire, he panicked and poured onto the burning oil the coldest thing he could think of...liquid nitrogen!!! Can you guess what happened?? Well, the same thing as above, only instead of the burning oil vaporizing the water, the burning oil vaporized the liquid nitrogen, which sprayed FIRE EVERYWHERE.

Now that you understand this concept, I will finish off with three neat applications.

One is a sawdust fire:

Sawdust itself won't burn that quickly, but when mixed with the right amount of oxygen, it will burn instantly. In that video it looks like they have a fan that mixes air with the sawdust, then they ignite the whole thing (it looks like they have a propane tank hooked up as well, maybe to make things a little crazier). Increasing oxygen concentration is the same reason you blow on your campfire to make it burn.

Second is a jet engine:

I think jet engines are cool because the big challenge is how to get enough oxygen mixed with the fuel to make it burn as quickly as possible. Pretty neat!!

And last but not least, EXPLOSIVES!!!

You've probably heard of nitroglycerin and TNT (trinitrotoluene):

These are good explosives because they don't need as much oxygen to explode because it has oxygen built in. The molecules have those "nitro" groups (composed of nitrogen and oxygen atoms) that can rearrange themselves into nitrogen and oxygen gas molecules. The nitrogen gas leaves, and the oxygen reacts with the rest of the molecule to form water and carbon dioxide gas. Since gases take up way more space than solids, you get a huge explosion.