Publication 7246

Martins I. P. & Cachapuz A. (1993) Making the invisible visible: A constructivist approach to the experimental teaching of energy changes in chemical systems. In: Proceedings of the Third International Seminar on Misconceptions and Educational Strategies in Science and Mathematics. Cornell University, Ithaca, 1–4 August 1993. Misconceptions Trust, Ithaca NY: **MISSING PAGES**. Fulltext at https://cepa.info/7246
The subject “energy of chemical reactions” has been referred/reported as a theme in which the students demonstrate several difficulties of an adequate understanding (Johnstone, 1980; Finley, Stewart and Yarroch, 1982; Granville, 1985; Lawrenz, 1987; Shaibu, 1988). Some alternative conceptions in this area have been identified and are discribed (Cachapuz and Martins, 1987; Martins, 1989). For example, high school students may think that in some chemical reactions one of the reactants may play a more important role than the other(s), the so called “principal reactant” (PR) (Cachapuz and Martins, 1988). The idea of “principal reactant” is probably a specific case of a more general difficulty on the part of students in perceiving a chemical system in its entirety and it may be considered as a contemporary version of the duality between the sulphur and mercury principles used by 13th century Alchemists to explain natural phenomena. As referred by historians of science (Caron and Hutin, 1964) the sulphur principle would explain the active and warm properties of materials (hence the idea of “principal reactant”) whereas the mercury principle would explain passive and cold attributes.

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