We report on two studies that assess to what extent sixth and seventh graders (a) did not differentiate between weight and density as quantities at the time of the preinterview and (b) made conceptual changes after participating in a curriculum on weight and density that uses computer-based conceptual models and simulations. In Study 1, 18 sixth graders received individual clinical interviews to assess their understanding of the density of materials and their ability to use a concept of density to understand flotation, immediately before and after they worked through an eight-lesson curriculum unit on those topics. In Study 2, 12 sixth graders and 10 seventh graders received individual clinical interviews before and after their classes worked through a more extensive 16-lesson curriculum on these topics and on the topic of changes in density with thermal expansion. In both studies, we found that the majority of students failed to differentiate consistently between weight and density at the time of the preinterview (although within this group, we distinguished between those who were beginning to have some insight that two distinct dimensions might be involved and those who were not), whereas the rest were able to make an initial qualitative distinction between these quantities. We found that our curriculum was moderately effective in helping some but not all children differentiate weight from density and -Requests for reprints should be sent to 222 SMITH, SNIR, GROSSLIGHT in helping other children consolidate their understanding of density. Further, the revised curriculum used in Study 2 seemed more effective than the curriculum used in Study 1. However, the curriculum was not equally effective for all groups of children (those children who had some initial insight that the two dimensions were necessary were helped the most). The experience of using the curriculum alerted us to the specific ways our models were helpful and certain complexities inherent in the process of using conceptual models to produce conceptual change. These complexities are discussed, along with ways to further revise the curriculum that might lead to its more widespread effectiveness with children.

ABSTRACT:Helping students understand the general nature of scientific models is increasingly regarded as an important goal of the middle and high school science curriculum (e.g., J. K. Gilbert & C. Boutler, 1998. International Handbook of Science Education; Kluwer, London; A. G. Harrison & D. F. Treagust, 2000. Science Education, 352 -381). In addition, beginning in middle school, students are introduced to one of the most central models in modern science-the particulate model of matter. Thus, teaching students about this model is an ideal opportunity to help students develop an understanding of the nature of models in the context of learning a central scientific concept-the discontinuity of matter. In this article, we present a software tool that was designed for this purpose. The software engages students with investigating and evaluating competing models of matter in order to help them see the particulate model as a plausible model that can explain a wide range of facts about diverse phenomena. The first and second parts of the paper describe the scientific content of the particulate model and the main ideas about scientific models that we would like to teach, as well as the educational challenges of teaching these ideas to middle school students. The third part describes the structure of the software and the three phenomena we chose to have students explore. These are all phenomena that should be puzzling to students if they assume that matter is continuous, but that can be easily explained if they assume that matter consists of discrete particles. The paper concludes with a description of two studies evaluating the effectiveness of the software in promoting students' understanding of models LINKING PHENOMENA WITH COMPETING UNDERLYING MODELS 795in general and the particulate model of matter in particular. We found that middle schoolers can engage with fundamental ideas about the nature of models, and that engaging them with these ideas helps them internalize the assumptions of the particulate model of matter. This happened especially for students who had developed relevant macroscopic conceptions of matter based upon quantified and interrelated conceptions of volume, weight, and density.

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of Oriented Helices 1653 provides a rationalization of the absence of ion pair formation in the isolated complex. It also permits the prediction that proton transfer and ion pair formation will occur between HC1 and a nitrogen base B if the proton affinity of the base exceeds about 225 kcal/mol.In a similar way, we can predict the ••• distances needed to form ion pairs in H3N-HBr and 3 complexes. Because of its low bond energy, HBr could form an ion pair at an ••• distance of 2.90 Á, which is 0.55 Á shorter than the 3.45-Á ••• distance in ammonium bromide (NaCl lattice). For HI, an ion pair could form at an ••• distance of 3.17 Á, 0.46 Á shorter than the 3.63-Á ••• distance in ammonium iodide (NaCl lattice). These contractions are quite close to that implied by the calculated •••0 distance in HaN-HCl, i.e., 0.40 Á shorter than in ammonium chloride. Hence we are led to the conclusion that proton transfer will occur in H3N-HI and that it might occur in HsN-HBr as well.16 ConclusionsThe matrix spectra show clearly that the HsN-HCl complex is not an ion pair, NH4+-C1_, but it can be re-garded as a strongly hydrogen-bonded complex with a proton shared by the adjacent heavy atoms. This is entirely consistent with the results calculated in an ab initio fashion by Clementi and it validates his application of selfconsistent field calculations without configuration interaction (electron correlation) to a hydrogen-bonded complex. Thus, the present work shows that the ionic character of the ammonium chloride crystals must be attributed to the long-range ionic lattice forces, without which ammonium chloride ion pairs do not form. It reassures, as well, the validity of ab initio calculations at the sophistication level used by Clementi and it should encourage such studies of other hydrogen-bonded systems that may not be experimentally accessible.(16)' Preliminary experiments with NH3-HBr codepositions produced spectra that include absorption near 1380 cm-1, a region in which NH4+ displays intense and characteristic absorption.