A brief review on the contribution to the knowledge of the difficulties and misconceptions in understanding the chemical equilibrium.

The existence and persistence of errors in the understanding of chemical equilibrium denotes student"s difficulties in learning and also in teaching methodologies. The knowledge of these difficulties, in depth, can help teachers to try to overcome them. This work is a review together with new contributions on the chemical equilibrium misconceptions analyzed by a group of university and high school teachers. On the one hand, we pretend to collect some of most common difficulties that students, from high school and university, show in re lation to chemical equilibrium and Le Chatelier's Principle application. On the other hand, we also intend to analyze the possible causes and even, ocasionally, make suggestions of activities to help teachers to improve the topic. This work collects difficulties and misconceptions related with basic concepts of equilibrium (system type, forward and reverse reaction rates, stoichiometry, catalyst function, equilibrium constant and reaction quotient, changes of equilibrium constant with temperature and concentration...); as well as with the understanding and the prediction of the direction of change in the equilibrium position by changing different variables such us composition, volume, pressure or temperature. Finally, educational implications addressed to the teachers have been underlined in order to amend the observed mistakes by alerting students and changing methodologies. In order to not excessively lengthen the study, ionic equilibria have not been addressed. Moreover, the work makes special emphasis on the most common and significant conceptual errors but, obviously, is not an exhaustive revision of all the possible misconceptions