Lisa Petrov, age 12, of Philadelphia, Pennsylvania, for her question:

Why does iron rust?

Scientists are not certain why some metals rust and some do not. But they can explain how a sturdy iron poker decays to a handful of rusty red flakes. They also know that this can happen only in certain conditions. When we know this, we can remove the rust making ingredients and protect our ironware from crumbling away.

A handsome new iron railing looks not at all like a pile of rusty flakes. But unless the iron is properly protected, this will be its dusty rusty destiny. For the ingredients that make it possible for iron to rust are right there in the moist morning air.

Ordinary iron is a chemical element, made from atoms that are all alike. We do not find quantities of pure iron in nature because its atoms are eager to combine with atoms of hydrogen and oxygen. Groups of these atoms form molecules of hydrated iron oxide, the flakey brown chemical compound known as rust.

The transformation o` iron to iron oxide is a sort of slow burning process called oxidation. It uses oxygen from the air and also water, preferably moisture from the air. This process of oxidation is slow, very slow. But after a few moist mornings, an unprotected outdoor iron fence shows rusty red spots.

If the rusting process is allowed to continue, in time the entire fence will become a pile of reddish flakes. No, the iron atoms have not departed. They now are locked into molecules of iron oxide. Each of these molecules is a package of iron and oxygen atoms, plus a molecule of water. And, as in most chemical compounds, this hydrated iron oxide is very different from its separate ingredients.

We now know that in order to rust, iron needs supplies of gaseous oxygen and also moisture. It is logical to suppose that we can prevent the rusting process if we shut off these two basic ingredients. This is true and there are many ways to foil the rust makers.

One method is to keep iron objects indoors where the air is too dry for the rusting process. But a lot of iron and steel must stay outdoors in bridges, railings and various buildings. The way to preserve this outdoor metal is to seal it from exposure to air and moisture. The seal may be a rust resistant paint of some kind.

Hoes and spades, patio furniture and many other small objects have parts of iron or steel. When they're left outdoors for weeks on end, air and moisture conspire to convert the iron atoms into rusty molecules of iron oxide. Of course, we can bring these objects indoors every evening. But it may be more convenient to coat them with grease or oil, which seals them from rust making conditions.

So far nobody has invented a rust resistant paint that lasts for years and years. This is why a great bridge needs a crew of painters, always on the job. When the job of covering all its iron and steel with a coat of rust proofing paint is completed, it is time to start repairing and repainting all over again.