Overview
In the last five years, Delaware has changed from a multi-stream 1 recycling system to a single stream recycling system. Multi-stream recycling requires the recycle-conscious citizens to separate the materials into categories; single stream recycling is sorted at a Material Recovery Facility (MRF). My students are familiar with the general concept of recycling because one of the collection locations is on school property as well as there being an ongoing initiative in our school to recycle waste produced onsite. Nevertheless, the students do not understand what happens to the recycling, the new method of collection for the single stream system and what is acceptable to recycle. Each of these concepts aligns with the state curricular expectations for seventh grade chemistry: density, solubility, and phase change (Appendix A). This unit culminates a four-week study of properties of matter using parts of the STC/MS Properties of Matter kit 2. This component will take two additional weeks.
The students struggle with the scientific concepts of density, solubility, and phase change because it requires abstract thinking to understand these ideas. At thirteen, only a small percent of students are beginning to think beyond the concrete. The behavior of microscopic particles in matter is often beyond most of my students' comprehension. If they cannot see it with their naked eye, they don't incorporate the concept into their understanding of how matter behaves. Though we draw models, interact with digital graphics, and experiment at the macroscopic level to image abstract or microscopic principles, the students still leave with misconceptions.
The majority of the students cannot get their minds around the idea that all matter is made of particles with space between these particles. They see something without 'holes,' whether solid or liquid and cannot believe that it has a specific degree of packedinedness - density. They are often able to calculate the density but do not tie it to how matter behaves or interacts. It is just some unconnected math problem. They do not tie the concept of density to floating or sinking though we calculate the density of materials and compare it to the density of water. We drop matter of different densities and sizes into a tank of water and watch it sink or float, noting the material's density. At least a handful of my students will complete the unit still stating that objects float or sink because of their size. From there it's all downhill: phase change and solubility hinge on the concept of particles and their interaction with one another. The major misconceptions are that a substance disappears, is consumed, or melts when it dissolves and that matter changes to a completely different substance when it changes phase.
Their misunderstandings are not limited to the scientific concepts but also recycling. Though most schools in our district have recycling programs, students are rarely taught what can and cannot be recycled and why. I watch students place used tissues and Styrofoam trays with food into the recycling receptacles. They do not realize that biological materials cannot be recycled at a MRF. Moreover, they do not question how the objects they discard are changed into material that can be reused.
The chemical and recycling misconceptions can be remediated at the same time. Single stream recycling requires an understanding of density to sort the materials. Solubility is the basis of the reclamation of paper, while phase change is the basis of the reclamation of metals, plastics, and glass. Biological matter contaminates the recycling process and renders much of the stream unusable.
To reinforce the basic characteristics of matter and alleviate mistaken beliefs, students will apply the concepts of density, phase change, and solubility while experimenting with methods of recycling in a single stream system. They will determine how to sort comingled recyclables to prevent contamination and how to reclaim the various materials for reuse. In this unit, students will explore the application of scientific concepts and techniques of investigation, problem-solving strategies, as well as behaviors that will help sustain the environment.
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