Teachers cannot help their students understand what they themselves do not understand.However, research has found that many high school chemistry teachers have a limited conceptual understanding of chemical equilibrium (Cheung, 2009a, 2009b; Cheung, Ma & Yang, in press).For example, chemistry teachers generally hold the misconception that one can always cause an equilibrium system to shift to the right by increasing the concentration of a reactant.Many teachers also have the misconception that adding an inert gas never affects the position of equilibrium.