There are inherent difficulties in a subject like chemistry particularly the notion of a chemical reaction. In this paper the difficulties are discussed from a teaching and learning perspective and from a history of chemistry perspective. Three teaching/learning studies of the incompleteness of the iron thiocyanate reaction in chemical equilibrium are reviewed and it is shown that a recent historical study of the iron thiocyanate reaction has the potential to challenge the interpretation of the incompleteness of the reaction. This establishes (...) a controversial context where students can be introduced to epistemic thinking, that is, how to interrogate chemistry data and form a conclusion which resonates with what we know about the nature of science. A curriculum suggestion for pre-service chemistry teachers is provided. (shrink)