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Living in a Material World
Oxide Films
Many metals will form an oxide film on their surfaces when they are in contact with air and/or water. For instance, rust is an iron oxide film on iron or iron-containing materials
e.g. steel.
A shiny copper roof will quickly go green with a copper oxide patina film.
The attractive colours of titanium jewellery are actually different thicknesses of titanium oxide films.
Aluminium is a very reactive metal and like many metals, when it comes into contact with air, it reacts rapidly with oxygen and is covered by a film of aluminium oxide (Al2O3). This protective oxide layer makes aluminium very inert (unreactive) and therefore safe to use as cooking foil.
Hydroxides e.g. sodium hydroxide (NaOH), can remove the oxide layer of foil, baring the reactive aluminium metal. The aluminium then reacts vigorously with the NaOH producing hydrogen gas (which can then be ignited, see photo).
Many cleaning products contain NaOH so you have to be careful not to use them on aluminium surfaces!
First the NaOH reacts with aluminium oxide:
Then NaOH reacts with the aluminium metal, producing hydrogen gas:
Aluminium foil & Sodium hydroxide - Hydrogen gas is given off & is ignited with a match
Next - Nitride Films
Previous - Electroplating
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