Introduce your students to the idea that different oxidation states of transition metal ions often have different colours and that electrode potentials can be used to predict the course of the redox reactions.

Zinc is used to reduce a yellow solution of ammonium vanadate(V) to a mauve solution containing vanadium(II) ions. The intermediate oxidation states of vanadium(IV) (blue) and vanadium(III) (green) are also seen.