Wednesday, October 19, 2011

Blue Steel, Or I Guess Red-Hot, Flaming Steel

Steel is not something that burns, right? In fact, fire doors are often made of steel for that reason. And everyone has seen something rust. Usually something you like, like your car. Rusting is a neat little reaction and it turns out that if it happens fast enough it can actually light steel on fire. And of course, if it is possible to light something on fire, particularly something odd and smelly, we here at Physics Buzz have tried it.

So what do rusting and burning have to do with each other? When steel rusts, some of the iron combines with oxygen from the air to make iron oxide. This reaction is an exothermic reaction, meaning it gives off heat. I’m sure you are saying to yourself that your rusted out Ford isn’t getting much hotter, just much grosser. Rusting usually happens very, very slowly so it isn’t easy to measure the change in temperature. If you could make rusting happen faster, say by soaking some steel wool in vinegar to remove the anti rust coating and then letting it sit for a minute or two, the temperature change is obvious.

Burning steel is pretty darn similar to rusting only much faster. To get something to burn, you have to first add a little heat energy such as a lit match to get the reaction started. Then, the material that is burning starts combining with the surrounding air quickly and releasing a whole lot of heat fast. That’s the fire.

Blocks of steel such as doors and nails can’t burn because there is not enough surface area to let enough oxygen in to get the reaction going. Sticking a match against a nail isn’t going to do all that much. But, if you have very thin ribbons of steel like in, say grade #00 steel wool which you can find and any Home Depot, there will be enough oxygen near enough iron to really get the reaction to go and the steel to blaze.

But lets say you don’t have a match or just want some other way to heat things up. When a lot of current passes through a thin wire, it will heat up pretty fast. Grade #00 steel wool available at any Home Depot is, conveniently, made up of a whole lot of tiny wires. A 9V battery can push a lot of current through these wires, heating them up pretty fast. Fast enough to make them catch fire.

So, don’t store your 9V batteries near your steel wool. Or, if you do, at least store them near your fire extinguisher too.