Written By: Felix Zhou
Fire. The ability to produce fire is one of the defining characteristics of humans, and it’s considered something that sets us apart from other animals. In fact, fire is so much a part of human society that you probably already have a pretty good idea of what can be set on fire and what can’t. Wood? Burn away. Sand? Forget about it. Paper? Up in flames it goes. Iron? Not so much… right?
Iron experiences oxidation, a chemical reaction where an atom loses electrons. In our minds, iron can’t burn, but it certainly oxidizes – most metals can. The most common form of iron oxidation is rust, when iron interacts with oxygen. You’ll probably agree that rust isn’t particularly spectacular. But the thing is, rusting releases energy, just really, really slowly.
Fire is also a type of oxidation reaction, but one that releases energy much faster, creating the scorching heat and spectacular light and flames.
Why Won’t It Burn?
So if iron oxidizes on the regular, why don’t we see people running around setting metal on fire all the time? Simply put, big blocks of metal react too slowly to end up with the rapid release of energy needed to start a fire. So they’ll only rust, and not burn, usually in a slow boring process that’s about as fun as watching paint dry. At best, even in pure oxygen, it might just glow a little bit. But why is it so slow?
Awesome Metal Flames
In a chemical reaction, molecules have to collide. A big chunk of iron has most of the material on the inside, with only a little on the surface to react, so the necessary collisions can’t happen fast enough to set it on fire, no matter how much heat and oxygen you throw in. But what about iron wool, or iron powder for that matter? These finely ground materials have much more surface exposed, so far more molecules can collide in a short time, making the same chemistry happen faster – fast enough to light iron wool on fire. And lo and behold iron burns.
Interesting side-note, what’s left of an iron fire is actually heavier than what you started out with. This is because the reaction actually forces oxygen from the air onto the metal as solid rust, instead of the thing on fire burning away.
Welding – No Electricity Needed
And it’s not just iron either. A lot of metals you wouldn’t think of as flammable burn in powdered form, with cool uses like welding. For example, aluminum powder can burn hot enough to fuse metal.
Isn’t it cool how us humans never fail to find more things to light on fire? I think so.