Intro to Chemistry - Case Notes

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Author - Dale Moore - Summer 1997

Protocol

1. Make initial demonstration: Add one squeeze (from dropper) of A (pink liquid) to each of five tubes. Add supplemental ingredients as follows: (1) Leave one tube as a reference; (2) add one squeeze of B (turns opaque, blue-green); (3) add one squeeze of D (turns blue on top); (4) add one squeeze of E (turns purple); (5) add one squeeze of C, then one squeeze of B (turns golden-red). Class must define "chemical combination." (Probably emphasize the formation of a NEW material as suggested by changes in color, opacity.)
2. May want to begin by asking students to consider how a scientist might determine that a chemical combination has occurred (formation of a NEW material). They should suggest observable properties (color, phase, temperature, boiling point, etc.), but should also come to the conclusion that any combination of observable changes may actually occur. including no observable chances at all.
3. Explain that the objective of the activity is to arrive at a "Theory of Chemical Combination." Several metaphorical MODELS (metaphors, proto-theories) are suggested below. Each small group may discuss one, all groups may discuss all four, or the class may discuss them as a large group. The discussions may be prompted by pointing to ISSLTES to compare: Order of combination, amounts of components combined, time elapsed, the possibility of interaction between components (for example, the flavor of parsley cancels the flavor of aarlic), etc. Want the students to ask which of these ISSUES may effect the result of a chemical combination. Combination models (metaphors): Combination of colors in mixing paint; Combination of ingredients in a soup (or other culinary product); Combination of sounds in music; Combination lock.Student Handout
4. Each group must report the results of their discussion. The board may be used to construct a table of MODELS v. ISSUES, indicating, how each issue will effect each model. (Try Listing MODELS in a vertical column on left-hand board; then add vertical columns for each ISSUE and cross-reference to fill-in expectations; use student handout as guide.)
5. Each group must propose experiments, indicating, the result predicted by each model (including, what observable results, from (2) above, would be expected for each tube). The experiments are conducted, and it is noted on the board (making notes or marks on the previously constructed table) which models aaree with the experimental results. [Optionally, the experiments are not conducted, and the groups must base their evaluations of models entirely upon the initial demonstrations.]
6. The class as a whole group must reach a consensual Theory of Chemical Combination.

Interpreting Results

• Order of combination: Doesn't matter for the given system. Example would be combining A and B, then adding C v. combining A, and C, then adding B. Same product results (golden-red solution).
• Time elapsed: Many students noticed in the trial session that for the combination of A, B, and C, the product material proceeded to darken over time. Thus elapsed time is a factor in chemical combinations. (That chemical combinations require an amount of time to proceed, even after mixing, may be made more evident by DILUTING, which would require the addition of water: Try one drop each of A, B, and C, then top-off tube with water. Another example: The rich blue-green solid from A+B fades to pale lilac color with time.)
• Amounts: Many students also noticed that the amounts of A and B combined caused the color of the product to vary (sometimes more greenish, sometimes more bluish). The droppers may be used to roughly measure amounts of components to combine, and the mass of precipitate (solid material) formed can be observed.
• Interaction: Adding C to A doesn't provoke a response (now color change; no apparent reaction), but adding C to the combination of A and B forms a NEW material. There are interactions between the components that make these chemical combinations more complex than a combination lock or the combination of pigments to make paint.
• Other combinations: Material A is actually a combination of water and cobalt; adding acetone (D) drives the water away from some of the cobalt, revealing the "cobalt" blue color of the water-free material; adding D to the other combinations (A+B and A+E) may also provide interesting- results.

Featured Concepts from Ziman:

• Consensibility: (Section 1.4) Each group must be able to communicate its ideas to the whole class.

• Consensuality: (Section 1.4) The class must reach a consensus in constructing a Theory of Chemical Combination.
• Models: (Section 2.5) The class will use metaphorical models to culde their analysis of the idea of combination, possibly constructing- their own metaphors or converting on a model of combination that derives from multiple metaphors.
• Theory/hypothesis: (Sections 2.5, 2.9) Ziman uses these two interchangeably (individuals may want to expand on the definitions). The exercise is meant to emphasize the interdependent nature of theory and experiment.
• Prediction: (Section 2.8) The class must test their theories by designing, experiments according- to their predictions.
• Faisifiablity: (Sections 2.5, 2.9) Necessary aspect of theories; part of the connection between theory and experiment.
• Observation: (Section 3. 1) As a segue into the next chapter of Ziman, the idea of the (non-) equivalence of observers may be introduced. In the trial, there was an amount of dispute about the observations of the initial demonstration that points to this important concept.