Ammonia water is a good precipitant that can react with various metal ions to form insoluble weak bases or amphoteric hydroxides. For example:

Al3++3NH3 · H2O==Al (OH) 3 ↓+3NH4+

The generated Al (OH) 3 precipitate is slightly soluble in excess ammonia water.

Fe2++2NH3 · H2O==Fe (OH) 2 ↓+2NH4+

The generated white precipitate is easily oxidized to form a reddish brown precipitate

4Fe (OH) 2+O2+2H2O=4Fe (OH) 3 (reddish brown)

By utilizing this property, Al (OH) 3, Fe (OH) 3, Fe (OH) 2 (benzene layer coverage), etc. can be prepared in experiments.