What are things made of and how can one explain their properties?
What changes and what stays the same when things are transformed?
How do we know?
For starters, even preschool children make some distinction between material and object levels of description and have learned words to label things at both levels: kinds of objects (e.g., boats, cars, beds, balls, kites) and kinds of materials (e.g., water, milk, play dough, wood, plastic). Children also have rich vocabulary for describing the properties of things based on commonsense impressions—for example, size (big/small), weight (heavy/ light),1 texture (soft/hard, rough/smooth), color (red/blue), shape (round/ square), taste (sweet/salty), smell—and have some initial ideas about which properties may pattern at object or material kind levels. They not only are fluent language speakers (which allows them to use language to form and express ideas in symbolic form) but they also have some facility at counting, drawing, and building or making things (which extends the resources they have for symbolizing things). Thus, they can use their existing ideas to engage in a variety of practices, including asking questions, describing and representing their observations, identifying and classifying things, making arguments, and proposing explanations (see the chapters in Part II for a review of research that supports these claims).
At the same time, the proposed learning progression acknowledges the extensive research that shows young children’s initial conceptual knowledge of materials, of physical quantities such as weight and volume, and of the knowledge construction practices of science are still quite limited. For example, although young children are learning names for some kinds of materials, descriptions at the level of objects is much more salient and important in their everyday life, and their knowledge of and experience with different materials is still quite limited (Krnel, Watson, and Glazar, 1998; Krnel, Glazar, and Watson, 2003).
Furthermore, although young children may implicitly treat materials as homogeneous constituents of objects in some circumstances, this understanding is still fragile and unarticulated. Indeed, there are many situations in which they deny that an entity broken into tiny pieces is still the same kind of stuff in part because it no longer looks like the same stuff (Dickinson, 1987; Krnel, Glazar, and Watson, 2003). Their knowledge of object properties is limited to those accessible to commonsense impressions, so many of
We have used “weight” rather than “mass” to describe the measured property of matter because it is the property that students measure and conceptualize at a younger age. We recognize that at some point students should learn to distinguish between weight as gravitational force and mass as a measure of the amount of matter. Even at the middle school level, though, this distinction probably is not critical for an understanding of atomic-molecular theory.