models: models were evaluated on the basis of their consistency with an entire pattern of results and their capacity to explain how the results occurred, rather than on the basis of a match with surface appearance. In this way, discussions of these simulations were used to help them build important metacognitive understanding of an explanatory model.

Describing and explaining the behavior of air or other gases provide still more fertile ground for demonstrating the concept that matter is fundamentally particulate rather than continuous. Of course, these investigations are effective only if students understand that gases are material, an idea that the proposed learning progression recommends they begin to investigate at the grades 3-5 level.

At the same time, coming to understand the behavior of gases in particulate terms should help consolidate student understanding that gas is matter and enable them to visualize the unseen behavior of gases. In other words, developing macroscopic and atomic-molecular conceptions can be mutually supportive. Direct support for this assumption was provided in a large-scale teaching study with urban sixth-grade students that compared the effectiveness of two curriculum units.9 One unit focused more exclusively on teaching core elements of the atomic-molecular theory, without addressing student misconceptions about matter at a macroscopic level. The other included more direct teaching of relevant macroscopic and microscopic concepts and talked more thoroughly about how properties of invisible molecules are associated with properties of observable substances and physical changes. The latter unit led to a much greater change in understanding phenomena at both macroscopic and molecular levels. Thus, sequencing instructional goals to reflect findings on student learning has important implications for how children make sense of science instruction.

Instruction that is focused on building core ideas is especially effective when students are regularly involved in classroom debates and discussion about essential ideas and alternative theories. Classroom debate and discussion make scientific experiments more meaningful and informative. Thus, building an understanding of atomic-molecular theory must also involve engaging students in cycles of modeling, testing, and revising models that describe a wide range of situations, such as explaining the different properties of solids, liquids, and gases, the thermal expansion of solids, liquids, or gases, changes of state, dissolving, and the transmission of smells.

Students engage in these types of discussions and investigations in the following case study.

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