HIGH SCHOOL AND UNIVERSITY STUDENTS’ UNDERSTANDING OF SOLUBILITY EQUILIBRIUM

Chemical equilibrium and associated concepts as dynamic nature of equilibrium, meaning of equilibrium constant, heterogeneous equilibrium or qualitative interpretation of equilibrium are fundamental in Chemistry. However, chemical equilibrium is a source of teaching-learning problems in High School and University as several research studies in chemical education have proved. Solubility equilibrium is not an exception and didactic research has shown evidences of the little learning achieved by High School students. Nevertheless, few studies have been developed at university level. Identifying which concepts, representations and mathematics, students learn well or with which ones struggle, can guide instruction and curriculum development. With this aim, we have examined students’ ideas about solubility equilibrium through a questionnaire. The questionnaire comprises five open-ended questions with different aims.

In this work, we discuss the learning difficulties related to the dynamic nature of the solubility equilibrium and its microscopic and macroscopic interpretation. A phenomenographic approach has been used in order to categorize the different ways the students perceive and understand reality (concepts and associated forms of reasoning). Our research was carried out with a random sample of 243 students from the last year of High School from different institutes in the province of Valencia, and 97 students who were studying the first year of Physics in the University of the Basque Country.

The data obtained indicate that students have problems to understand which the way for chemical equilibrium to be achieved is. It is noticeable that few students are able to explain that the equilibrium is reached when both the precipitation and solubility reaction rates are equal (less than 10% in High School and less than 25% in University). Some students think that chemical equilibrium is produced while the solute is dissolving and finishes when no more solute can be dissolved (which is really when equilibrium takes place). Other ones do not even consider that an equilibrium exist or have a naive macroscopic vision in which they speak about the quantity of salt that can be “accommodated” in the solvent. Most worrying of all is that many students could not give an answer, especially in High School sample.

Regarding the dynamic nature of the equilibrium, correct answers were scarce. Most of the students make use of the solubility concept to explain it but showed different mistakes in their explanations. Remarkably some of the students misunderstand chemical equilibrium with a static position.

We can conclude that in spite of years of study, students’ ideas about solubility equilibrium are far away from scientific concepts. Students do not have a microscopic understanding of chemical equilibrium; instead, they rely in what they can see when dissolving a solute and use a macroscopic model to explain it. Thus, our findings can be useful to have a prior knowledge of the problems that students must face in the study of these concepts and to prevent possible mistakes by taking them into account during teaching activities. Indeed, a second phase of our research can take us to develop a teaching sequence design for introductory chemistry courses at secondary school and university taking into account the students’ difficulties in order to improve the teaching-learning process.