The composition of aluminum foil is NOT 100% pure aluminum, as many people believe. You may have noticed that aluminum foil always seems dull on one side, and shiny on the other. This is caused by a naturally-occurring reaction. When Aluminum comes into contact with oxygen, the two elements undergo a spontaneous reaction:
4 Al(S) + 3 O2 (G) => 2 Al2O3 (S)
The aluminum foil reacts with oxygen in the air to form a thin layer of Aluminum Oxide on one side of the foil. Because rolls of aluminum foil are generally rolled tightly, the inner side of the aluminum foil is not exposed to the air as much, and it remains shiny.
In short, the composition of aluminum foil is pure aluminum (Al) AND aluminum oxide (AL2O3).
(And don't worry-- the layer of aluminum oxide on aluminum foil is completely harmless,like the aluminum foil container. We've been using foil to cover food for years, after all.)
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