Have you ever burnt a stack of papers? Maybe a few exams you didn't do so well on in that high school history class and you thought you'd avoid a load of grief from your folks by just burning the evidence?
Well, if you weren't doused with water by a parental unit hiding in the bushes, what happened to those papers when you lit them up?
Smoke And Ash And...
Of course burning paper turns it to smoke and ashes and from our everyday experience, whether it's burning old exams or firewood, we typically experience burning in a similar way: when we burn something it gets lighter or, in other words, it loses mass. But this is not true in general!
For example, when metals are burned, they actually gain mass!
This is because combustion, another word for burning, is actually a chemical reaction which produces heat via a reaction between an oxidant, like oxygen, and some other substance.
For example, when paper is burned oxygen from the air combines with carbon and hydrogen in the paper turning some of it into carbon dioxide and water vapor, which waft away with carbon particulates in the smoke. This, not surprisingly, leaves the solid ash leftover lighter than the original paper.
However, burn a common metal such as steel, copper, or aluminum and the oxygen from air actually sticks to the metal forming an oxide instead of producing heavy smoke, making the whole mass heavier.
When iron, or any other metal for that matter, rusts a similar process is taking placethat is, oxygen from the air is bonding to the metal, increasing its mass.