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Definition: Have a look at these nails. You can see there are two different groups: one group looks clean but the other is dirty – they're rusty. Only two metals can rust – iron and steel. The reason the surface of the rusty nails looks dirty is that they have been left outside. They've got wet. Water and oxygen have reacted with the surface of the metal to start a reaction. Bits have gradually fallen off the nails - that's rust.
reproduced by permission from Hodder