In grades 6-8, building on robust learning experiences in the lower grades, students are ready to make a fundamental conceptual leap. They are ready to explain a host of new phenomena, and to reexplain phenomena they are already familiar with, using a new understanding of atoms and molecules. This new understanding will enable them to distinguish between elements and compounds. They can begin to recognize other considerations in tracking the identity of materials over time, including the possibility of chemical change. Some transformations involve chemical change (e.g., burning, rusting) in which new substances, as indicated by their different properties, are created. In other changes (e.g., changes of state, thermal expansion), materials may change appearance but the substances in them stay the same. Students can describe and explain the behavior of air or other gases. In general, they come to appreciate the explanatory power of assuming that matter is particulate in nature rather than continuous.

The learning progression proposes that, during these grades, students can be introduced to the following core tenets of atomic-molecular theory:

• Matter exists in three general phases—solid, liquid, and gas—that vary in their properties.

• Materials have characteristic properties, such as density, boiling point, and melting point.

• Density is quantified as mass/volume.

At the microscopic level:

• There are more than 100 different kinds of atoms; each kind has distinctive properties, including its mass and the ways it combines with other atoms or molecules.

• Each atom takes up space, has mass, and is in constant motion.

• Atoms can be joined (in different proportions) to form molecules or networks—a process that involves forming chemical bonds between atoms.

• Molecules have characteristic properties different from the atoms of which they are composed.