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Hank Green: So we all know that you can get metal hot enough to melt and fold or be hammered into meter-long double-edged claymores, but can you actually burn it?  You probably know metals from their most familiar forms, a gold ring, a fireplace poker, a soda pop can, but metals are actually a pretty diverse group.  In fact, they make up most of the elements in the periodic table.  Metals are generally defined by the physical traits we associate with them: they're hard, but malleable, meaning they can be hammered into thin sheets, but they're also ductile, or can be pulled into wires, but probably their most important well-know property is that they're awesome conductors of heat and electricity.  They're also really generally grey and shiny.  And the reason they're so good at conducting electricity is basically the same reason that some of them can burn.  

Atoms of most metals tend to give up electrons really easily, so when heat or electricity run through a piece of metal, that's basically a stream of excited electrons, and when they burn, metals are giving up some of their electrons, but the process doesn't always look like you might imagine.  A hunk of pure iron, for example, doesn't tend to stay pure for very long.  That's because those iron atoms really want to hang out with oxygen.  Oxygen loves electrons, it is more than happy to take up all the precious electrons that iron is eager to surrender.  So, when exposed to oxygen, either in air or water, the iron rusts and becomes iron oxide.  So what does that have to do with burning?  
Well that is a chemical reaction called oxidation.  Burning is also oxidation, just much faster.  So rusting is a kind of burning, it actually does give off a little bit of heat, it's just very slow and doesn't actually ignite anything.  But iron can ignite, under the right circumstances.  The hotter the iron, or really any metal gets, the greater its affinity for oxygen becomes.  So if you get small enough pieces of it hot enough, it'll reach a point where it undergoes what's basically very rapid oxidation.  This point is its ignition temperature.  

You can try it at home, all you need is some extra fine steel wool, some pliers, and like a lighter or something that makes fire, and something that you can catch the hot flakes of metal in, so that they don't fall on your counter and catch on fire.  Be careful.  This steel wool is basically just thinly shaved metal, it's mostly iron and the more fine, the more surface area the metal area has, the easier it is to burn.  Let's see if we can make this happen.  Oh, wow, that's pretty.  It works.  And did it hit me in the face with a little bit of pain?  Yes.  

So why will steel wool burn but not the Brooklyn Bridge?  Or a cast-iron skillet?  Well, for a few good reasons.  First, because metal is such an excellent heat conductor, it disperses heat that's applied to it, which makes it harder for any particular part of it to reach its ignition temperature.  Put a skillet on a flame and the heat quickly disperses through all of the metal, so the surface doesn't get hot enough to sustain a reaction that would actually set the whole skillet on fire.  But those tiny little threads of steel wool are so thin that there isn't any room for the heat to spread out, so the iron quickly reaches its ignition temperature.  A second major factor has to do with surface area.  Most of the iron in this skillet is inside the skillet.  It doesn't come into contact with the air.  Steel wool burns because these long strands have huge surface areas that are exposed and ready to react with the oxygen in the air.  It's like a piece of paper is super easy to light on fire, but a big old log is hard to get going.  Iron filings will burn, iron rods will not.

In the end, it's a good reminder of how everything boils down to particles either reacting or not reacting.  When you visualize metal as a cast iron skillet, you only see it deflecting flames and never actually burning.  But if you think of it in its elemental state, remember how iron and many other metals have big old crushes on oxygen, it'll broaden your understanding of pretty much everything.

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