So far, we have evidence that significant restructuring has taken place in the knowledge network of the majority of the Treatment students by the end of 4th grade but not the control group. Weight, was initially measured by hefting conceived as a property of both objects and materials (i.e., “heavy” had the same meaning in “iron is heavy” and “this big cylinder is heavy”), and not tied to matter (some materials, e.g., Styrofoam, weighs nothing; small pieces of stuff weigh nothing). It is now a property of objects, or rather of the amount of material an object is made of (as demonstrated by students’ judgments that weight does not change when a clay ball is turned into a pancake), measured with a scale, and an inherent property of any piece of solid or liquid material, however small (as when they argue “everything has weight and takes up space”). Weight and size are both taken into account when thinking about the weight of objects made of different materials—objects made of brass are heavier for their size than objects make of aluminum. A concept of volume as “occupied 3D space” is now part of the network.
In keeping with the learning progression approach, we believe attention to restructuring students’ knowledge network prior to introducing phase changes, the idea that matter includes gases, and a formal treatment of density is critical if these more advanced ideas are to be meaningfully integrated into students’ knowledge network and hence to move it forward toward the atomic molecular theory without destabilizing it. The data we gather with these same students in fifth grade will be a critical further test of this assumption. We can see why longitudinal studies, though hard to do, are so important. If the Inquiry Curriculum is indeed “preparing the ground” in robust ways, students will be able to integrate the idea that gases as matter into their matter knowledge network—a notoriously counter-intuitive and challenging idea for students.