The aim of this laboratory exercise is to present a high school hands-on laboratory experiment, focused on obtaining and investigating the properties of various polymorphic forms of aluminum oxide. Amphoterism plays a key role when discussing the law of periodicity and periodic changes of acid-base properties of elements and their compounds. In many school syllabuses, that issue is based on third period elements, so the amphoteric substances are aluminum, aluminum oxide, and aluminum hydroxide. Although demonstrating those properties for aluminum and aluminum hydroxide is relatively simple, the same reactions for alumina can be quite problematic. It is due to the fact that aluminum oxide exists in several polymorphic forms, and only one of them reacts with acids and bases. Moreover, the commercially available aluminum oxide materials are usually nonamphoteric. The exercise describes preparation of various polymorphic forms of aluminum oxide by thermal decomposition of aluminum hydroxide, and tests for its acid-base properties. The experiment can be used as a separate activity, or as a part of a larger scenario considering the properties of third period oxides. It was successfully conducted with over 100 high school (K-11) students studying chemistry at standard and higher level according to the International Baccalaureate Diploma Programme.