Why does steel wool burn, when bigger bits of steel like nails, screws, knives and forks don’t?
It’s a combination of two factors: more availability of oxygen and easier build-up of heat. In a lump of steel, most of the iron is inside the lump. When the steel is in very fine strands, there’s a much bigger surface area of steel in contact with the air. That means more iron is available to combine with the oxygen in the air, so a lot of heat builds up in a small area. In fact, the steel stops burning when iron oxide builds up enough to stop oxygen getting to any more of the steel. There may well be some iron inside each burnt strand, but it can’t react because no oxygen can get to it there. This effect of surface area effect is actually a very general principle. For example, it’s hard to set fire to wood when it’s a big log, but very easy when it’s in the form of paper. The extreme case is a very fine powder. So much of the material is in contact with the air that reactions can happen very quickly. There have been several incidents of explosions in paper mills and flour factories a
