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student was instructed to firstly heat a piece of copper wire and, secondly, a piece of magnesium strips. Following each experiment, the student was then asked to describe and explain the outcome of the experiment in terms of both the macroscopic and microscopic changes associated with it. Questions were also raised concerning the conservation of mass, the reduction of copper oxide (how can we get back copper?), and the energy transformations due to bond braking and reforming. The results of the analysis showed that the students had limited understanding of the oxidation of copper, the combustion of magnesium, and the chemical reactions in general. Even when they described the phenomena in scientific terms, many could not understand the formation of copper oxide and considered that the black colour was a result of the flame from the Bunsen burner. Other students thought that the mass of the copper wire would be less after heating, because oxygen, which as a gas is lighter, replaced copper. Many students expressed the idea that the “combustion” of magnesium was a physical change that resulted in changes in the appearance of the magnesium strip, and thus the mass remained unchanged. These students and others did not consider the “invisible” oxygen to take part in the process. The majority of the students insisted that in chemical reactions only one of the reactants play an active role. Many students considered the oxidation of copper and the combustion of magnesium to be endothermic rather than exothermic reactions, because the metals were initially heated. Only two students proposed correctly reduction as a means to get back copper, but only one of them gave correct explanation. Taking into consideration the results of the study and the importance of oxidation and combustion as biological processes and as basic concepts in chemistry, and even physics, recommendations related to curriculum and teaching are presented. These recommendations stem from the point of view of learning as a construction process rather than a process of information delivery. The need to encourage action research and employ teaching interventions that facilitate students conceptual understanding and development is also addressed.

Key words: Critical thinking, epistemological development, educational interventions, science education.

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