Teaching for Understanding: A Study of Students' Preinstruction Theories of Matter and a Comparison of the Effectiveness of Two Approaches to Teaching About Matter and Density

Abstract Thirty 8th-grade students were given an interview and a written test before and after a 10-week curriculum unit concerning matter, mass, volume, and density. The instruments probed qualitative understandings of matter; ability to differentiate weight and density using qualitative reasoning; formal, quantitative understandings of mass, weight, volume, and density; and ability to integrate both qualitative and quantitative reasoning about density. In Part 1 of the study, we examined the organization of student ideas prior to instruction. We found evidence to support our idea that students' qualitative conceptions of matter and density were organized in commonsense theories of matter that constrained their understanding of density: Students who believed that all material objects have weight, no matter how small or light the object, were much more likely to have made a beginning differentiation between weight and density than those who did not. We also showed that a qualitative understanding of density emerged prior to a formal, quantitative understanding of density, although most students were able to engage in explicit proportional reasoning about another, more familiar quantity (i.e., sweetness). In Part 2, we compared the effectiveness of two teaching approaches. One approach to teaching about matter and density-the standard Introductory Physical Science (IPS) curriculum-emphasized formal definitions, measurement, and explicit quantitative reasoning. Because it asked students to work formally with complex concepts before assuring that relevant qualitative understanding was in place, we designed a modified IPS curriculum that addressed this problem by encouraging students to make their initial assumptions about matter explicit and open to debate. The modified curriculum also used visual models and qualitative reasoning to help students bridge the gap between their starting conceptions and formal, quantitative definitions. We found that both curricula were effective in promoting a good quantitative understanding of mass, volume, and density. The modified cumculum, however, was more effective at restructuring students' qualitative conceptions and at promoting an integrated understanding of density. Thus, we conclude that science curricula should integrate both qualitative and quantitative reasoning to be effective at promoting conceptual change.