Students' conceptions of chemical change



One hundred high school chemistry students who had completed a unit on chemical change were given a written instrument in which they were shown three oxidation-reduction reactions and were asked to explain them. Eleven students representing a range of achievement levels were chosen for more intensive clinical interviews in which they explained their responses, evaluated the quality of their responses, and compared them to other hypothetical responses. Interview results revealed that students commonly experienced difficulties at three different epistemological levels:

1. Chemical knowledge. Most students failed to invoke atoms and molecules as explanatory constructs, even though they had been emphasized in their chemistry course. Some students also listed “substances” such as heat, cold, or decay as reactants or products.

2. Conservation reasoning. Many students could not predict or explain mass changes in the chemical reactions. Their most common problems included (a) a tendency to treat chemical changes such as rusting as physical changes in form or state, and (b) failure to understand the role of invisible (in this case gaseous) reactants or products in the reactions.

3. Explanatory ideals. Many students demonstrated a preference for explanations based on superficial analogies with everyday events (e.g., rusting is like decay) over explanations based on chemical theories.

Only one of the 11 students interviewed demonstrated mastery of the unit's contents at all three levels. Results of this and other research indicate a need for substantial revision in chemistry teaching practice.