Corrosion
Shiny element that is a good conductor of electricity and heat, and which forms basic oxides. can The gain of oxygen, or loss of electrons, by a substance during a chemical reaction. in air. They react with oxygen and form metal oxides. For example, sodium is a very The tendency of a substance to undergo a chemical reaction. 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 鈫 sodium oxide
4Na(s) + O2(g) 鈫 2Na2O(s)
Other metals may oxidise more slowly. Gold and other very unreactive metals do not oxidise in air at all.
The destruction of a metal by oxidation or chemical action, eg rusting. happens when a metal continues to oxidise. The metal becomes weaker over time, and eventually all of it may become metal oxide.
Rusting
A corrosion process in which iron or steel reacts with oxygen and water. occurs when iron or steel reacts with oxygen and water:
Iron + oxygen + water 鈫 hydrated iron(III) oxide
Hydrated iron(III) oxide (rust) is the orange-brown substance seen on the surface of rusty objects.
A rusting experiment
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:
- in the middle test tube, where there was water but no oxygen (because there was no air in the water)
- in the right-hand test tube, where there was oxygen (air) but no water
Worked example
Explain whether iron is oxidised or The loss of oxygen, gain of electrons, or gain of hydrogen by a substance during a chemical reaction. when it forms rust.
Iron is oxidised because it gains oxygen during rusting.