A problem about solubility equilibrium is presented as a resource for the evaluation of students. It involves macroscopic, microscopic, and symbolic levels of representation, and allows one to assess whether students have acquired an adequate conceptual understanding of the phenomenon. The problem, which starts with a figure using numbered particles, is useful for judging the relationships that students establish among the three levels. The difficulties students encounter while solving the problem are discussed.

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This paper presents a thorough literature review of the analogies used to teach chemical equilibrium. The main objective is to compile all the analogies that have been found to be of service to the teacher and the student. Additionally, we categorize and analyze analogies in relation to the following aspects: representation of the dynamic nature of equilibrium, the equality of the rates of forward and reverse reactions, the reversibility of the reaction as the concept involved, the calculation of the equilibrium constant for a reaction, the application of Le Chatelier's principle; and the function of a catalyst in an equilibrium system. Some issues related to the use of analogies in teaching and learning are discussed, as are the misconceptions that can be erroneously introduced, reinforced or avoided with their use. Finally, some advice is provided about how the knowledge gained from this review could benefit practice.

This article presents and discusses the results of a study that investigates university students’ comprehension of the concept of molar concentration, following teaching and evaluation of the subject. The specific problems underlying learning of this concept have not yet been focused on in sufficient detail or depth. A “Reasoning with molarity” questionnaire, which explores the logical–mathematical relationships between the variables n, V and M, was administered to 303 Argentine first-year university students. Results obtained from think-aloud interviews related to this questionnaire, which were performed with 18 students, are also analysed. These results reveal that approximately half the first-year university students have no clear conceptual understanding of molarity. The main difficulty arises in inverse proportionality tasks where the number of moles (extensive) and M (intensive) should be related qualitatively to determine which solution occupies the smallest volume. It was also established that much conceptual confusion, such as the lack of differentiation between n and M, lies behind the algorithmic, numerical solution strategy carried out exclusively by many students.

Models, analogies and metaphors are strategies for constructing the knowledge because they construct, compare and evoke images. Models, analogies and metaphors are frequently used in chemistry teaching although of a non conscious or planned way. In this article, the relationships between models, analogies and metaphors are presented. Some conclusions about teaching with them are also showed.