5-8

E7: Investigate the similarities and differences between elements, compounds, and mixtures.

Curriculum Organizing Questions	What is the difference between a chemical symbol and a chemical formula? How can mixtures be separated (simplified)? How can compounds be simplified? What are the differences between a physical and a chemical change?
Elaboration	Elements can not be simplified physically or chemically. Compounds (made of 2 or more elements bound together) can be simplified chemically but not physically. Mixtures (made of 2 or more elements or compounds intermixed but not bound together) can be physically separated (simplified). Physical properties can be measured without changing the properties of the material being tested or making new substances. Chemical properties are measured by changing a material's properties and making new substances.
Specific Ideas	Chemical elements do not break down during normal laboratory reactions involving treatments as heating, exposure to electric current, or reaction with acids. NSES B1c A mixture of substances can often be separated into the original substances using one or more characteristic properties. NSES B1a
Developmental & Instructional Implications	It can be very difficult for students to classify a material as an element, mixture, or compound unless they have had laboratory experience with attempting to simplify it. (Preview Committee) When students first begin to udnerstand atoms, they cannot confidently make the distinction between atoms and molecules or make distinctions that depend upon it - among elements, mixtures, and compounds, or between "chemical" and "physical" changes. Benchmarks p. 77 By the end of 8th grade, students should have sufficient grasp of the general idea that a wide variety of phenomena can be explained by alternative arrangements of vast numbers f invisibly moving, tiny parts. Benchmarks p. 77 Students should see a great many examples of reactions between substances that produce new substances very different from the reactants. Then they can begin to absorb the rudiments of atomic/molecular theory, being helped to see that the value of the notion of atoms lies in the explanations it provides for a wide variety of behavior of matter. Benchmarks p. 78 In grades 5-8,the focus on student understanding shifts from the properties of objects and materials to the properties of the substances from which the materials are made. NSES p. 149 In general, pupils find difficulty in developing an adequete conception of the chemical combination of elements until they can interpret combination at the particle level. However, a particulate view does not ensure an understanding of chemical combination. Driver p. 97
Examples

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