The Fire That Takes a Year

Hold a thin iron wire in a hot flame and it does something most people have never seen iron do. It glows white, throws sparks, and in about four seconds it is gone — replaced by a dark, crumbly powder that is no longer metal. Iron burns.

Now go and look at an old iron gate that has stood in the rain for two years. It is covered in a reddish-brown crust that flakes off in your fingers. It is no longer metal either.

Those are the same event.

In both cases iron met the oxygen in the air and handed itself over. In both cases the metal is gone and an oxide is left. In both cases heat came out. The only difference between the two — the only one — is that the first took four seconds and the second took about two years.

That is a factor of roughly fifteen million. And a factor of fifteen million in speed is enough to make people give the two things different names, put them in different chapters, and never notice they are watching the same thing twice.

Rust is a fire burning too slowly to see.

If that sounds like a nice line rather than a fact, here is how to check it. Chemical hand warmers — the sachets people carry in cold weather — contain iron powder, a little salt, and sawdust. You tear the outer wrapper, air reaches the powder, and the sachet gets hot, hot enough to be uncomfortable, and stays hot for hours.

There is nothing in that sachet but rusting iron. The manufacturer has simply arranged for the rust to happen in an afternoon instead of in two years, and the heat that a gate gives out so slowly that nobody can feel it comes out fast enough to warm your hands.

Rusting was always giving off heat. You had just never seen it concentrated.

By the end of this lesson you'll be able to:

• tell a chemical change from a physical change, and give the sign that settles it
• name the everyday signs that a chemical change has happened
• explain rusting, burning and rotting as one reaction at three different speeds
• explain why cooking belongs on the list even though oxygen plays no part in it
• say what rusting needs, and therefore how to stop it