Aluminum has one interesting and very useful property. In moist air, it combines slowly with oxygen to form aluminum oxide:

The aluminum oxide forms a very thin, whitish coating on the aluminum metal. The coating prevents the metal from reacting further with oxygen and protects the metal from further corrosion (rusting). It is easy to see the aluminum oxide on aluminum outdoor furniture and unpainted house siding.

Aluminum is a fairly active metal. It reacts with many hot acids. It also reacts with alkalis. An alkali is a chemical with properties opposite those of an acid. Sodium hydroxide (common lye) and limewater are examples of alkalis. It is unusual for an element to react with both acids and alkalis. Such elements are said to be amphoteric.

Aluminum also reacts quickly with hot water. In powdered form, it catches fire quickly when exposed to a flame.

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