History of Chemistry as a Part of Assessment of Students' Understanding of the Law of Conservation of Mass

The aim of this research was to explore students' ideas about chemical reactions and difficulties in understanding the law of conservation of mass in such reactions by using an approach that started from presentations of scientists' work associated with the law. The developed test items relied on: 1) the historical contents that illustrate the experimental work of three scientists (Lavoisier, Landolt and Lomonosov); 2) the description of school experiments and 3) real life situation. In this way, students would have an opportunity to show understanding of the law of conservation of mass in two contexts, one based on the stories from the history of chemistry and the other contemporary, based on school laboratory experiments and real life situation. Students of different ages were selected for the research: the seventh and the eighth grade of primary school (age 13-14), and the second year of grammar school (age 16). The research involved a total of 301 students. The results indicated that students' difficulties were mostly associated with the predictions and explanations of mass changes in open reaction systems in which a gas was a reactant than with the reactions in which a gas was a product