In Baeyer's process the powdered bauxite is leached with

Alumina in bauxite is amphoteric and dissolves in hot concentrated NaOH to give sodium aluminate, $\mathrm{Na}[\mathrm{Al}(\mathrm{OH}){}_4]$, while iron oxide impurity does not dissolve. Cyanide is used for silver and gold, and water does not dissolve alumina.