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What is Rust? - Combustion Reactions
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What is Rust?

Rusty fence
(Thanks to Wikipedia)

Fire isn't the only kind of combustion reaction. Some kinds of metal, if you leave them in contact with oxygen, will also react with the oxygen, but much more slowly, and without making a flame. We call the result rust.

Like burning grass, the combustion reaction between iron and oxygen that produces rust, or iron oxide (a molecule containing both iron and oxygen atoms), also produces heat. In fact, it produces the same amount of heat as the fire, but spread out over a longer time, so that the iron oxide or rust also cools off at the same time that it is heating up.

When iron is exposed to oxygen in the air, the two combine very slowly to make iron oxide, or rust. But if the iron and oxygen molecules are also touching water molecules, the water molecules speed up this reaction and make the iron rust a lot faster.

Exekias vase
Black-figure vase by Exekias (Athens, ca. 540 BC)

One special kind of rust reaction happens inside a pottery kiln when you heat up a clay pot and there is some iron in the clay. The iron in the clay reacts with the oxygen inside the kiln and turns red. This is how Greek vase-painting worked.

Learn by Doing - Rust
Another slow kind of combustion reaction: digestion
A faster kind of combustion reaction: fire

Bibliography and further reading:

Fire
Iron
Chemistry
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Professor Carr

Karen Eva Carr, PhD.
Assoc. Professor Emerita, History
Portland State University

Professor Carr holds a B.A. with high honors from Cornell University in classics and archaeology, and her M.A. and PhD. from the University of Michigan in Classical Art and Archaeology. She has excavated in Scotland, Cyprus, Greece, Israel, and Tunisia, and she has been teaching history to university students for a very long time.

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