One reason students find it difficult to learn the particle model of matter is that traditional curriculum materials present concepts to students without helping them to develop these ideas. The How can I smell things from a distance? sixth grade chemistry unit takes the approach of building students' ideas through their construction and revision of models. Progress variables have been proposed as a means to address the need for curriculum and assessments that can help teachers' improve their practice as well as to inform both students and teachers about students' performance. Progress variables depict students' increasingly sophisticated conceptions of a specific construct during instruction. This study provides evidence that curriculum and assessment based on modern learning theories, can lead to the development of progress variables that are able to track middle school students' understanding of the particle nature of matter over time. This study used a progress variable to track student understanding of the particle nature of matter during the sixth grade chemistry unit.

I describe the assessment system used to develop the progress variable for tracking students' development of particle model of matter during the sixth grade chemistry unit. A calibration study determined that the chemistry unit's assessments were reliable and valid measures of the particle model of matter progress variable. Further analysis revealed that the progress variable had to be modified such that the levels were more distinct.

The modified progress variable was empirically validated so that it could be used to track students' understanding during instruction. Results indicate that a validated progress variable, linked to coherent curriculum and assessments can provide valid interpretations of students' knowledge of particular domain during instruction and that this progress variable is valid for students from varying populations and backgrounds. In addition, well-aligned curriculum and assessment can provide insight into instructional sequencing that help students in developing a particle model of matter. Results also indicate that students are able to reach a completely particle view of matter. These results hold implications for curriculum development as well as for understanding the ways in which students develop a particle view of matter.