Surprisingly, spectator ions are responsible for unexpected kinetics in the biphasic copper(II)-aluminum displacement reaction, with the rate of reaction dependent on the identity of the otherwise ignored spectator ions. Application of a published kinetic analysis developed for a reaction between a rotating Al disk and a Cu(II) ion solution to the simple reaction using aluminum foil and a copper solution is demonstrated. The chloride-acceleration effect is found to be true for both chloride and bromide and should be more generally termed a halide-acceleration effect. This interesting and simple reaction can be used to deepen students' understanding of biphasic processes that are important industrially, that is, aluminum corrosion, and to "burst the bubble" of rigid thinking that spectator ions always can be ignored. (Contains 1 table and 2 figures.)