Metals can oxidise in air. They react with oxygen and form metal oxides. For example, sodium is a very reactive metal. When sodium is cut or scratched, its freshly exposed shiny surface rapidly turns dull as a thin layer of sodium oxide forms:
sodium + oxygen → sodium oxide
4Na(s) + O2(g) → 2Na2O(s)
Other metals may oxidise more slowly. Gold and other very unreactive metals do not oxidise in air at all.
Corrosion happens when a metal continues to oxidise. The metal becomes weaker over time, and eventually all of it may become metal oxide.
Rusting occurs when iron or steel reacts with oxygen and water:
iron + oxygen + water → hydrated iron(III) oxide
Hydrated iron(III) oxide (rust) is the orange-brown substance seen on the surface of rusty objects.
The experiment in the diagram shows that both oxygen and water are needed for rusting to happen.
The nail only rusts in the left-hand test tube. It does not rust:
Explain whether iron is oxidised or reduced when it forms rust.
Iron is oxidised because it gains oxygen during rusting.