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In “A-B-C Pipettes,” students must use deductive and “compound” reasoning to solve a chemical mystery
9, 10, 11, 12
Title – A-B-C Pipette
By – Bonnie Buddendeck
Primary Subject – Science
Grade Level – 9-12
A, B, C Beral Pipettes
Each Beral-type pipette labeled A, B, or C contains a dilute solution of calcium chloride, sodium carbonate, or hydrochloric acid. Using only these three solutions, determine which solution is in which pipette. Describe the method you developed to solve this problem.
This problem tests a student’s ability to use simple deductive reasoning to solve a chemical problem. Students must demonstrate their knowledge of some simple chemical properties of common compounds, and also that they can interpret their results to successfully identify the three solutions.
ONE LIKELY APPROACH
1. The student devises a plan so that each solution is reacted in a systematic way with the others, and records in a clear manner. For example, the reaction matrix might look like this.
2. The student observes, for example, that A and B unknown solutions react to produce a gas. If this happens, it must mean that they are HC l (aq) and Na 2 CO 3 (aq), although which is not yet apparent.
3. The student identifies the third solution, C, with each of the others, and observes which combination produces the precipitate.
4. The student identifies the third solution, C, as CaC 12 , the reacting solution, A, as Na 2 CO 3 . HC l is therefore the remaining solution, B.
Note: This set of observations is based on a set of unknowns where A is Na 2 CO 3 , B is HC l and C is CaC l2 .
Your selection of unknown orders may vary not only from class to class, but even within the class for more variety.
SCORING SUGGESTIONS (BASED ON 5 POINTS)
1. (2 pts) Initial checking for reactions between each solution.
- a) (0 pts) The student does not test the solutions with each other.
- b) (1 pt) The student tests each solution with each other solution, but fails to record observations clearly or completely.
- c) (2 pts) The student uses the likely approach procedure.
2. (3 pts) Presentation of evidence for identification of each solution.
- a) (0 pts) The student does not present any reasonable evidence to support identification.
- b) (1 pt) The student presents some reasonable evidence to support identification.
- c) (2 pts) The student presents significant evidence to support identification.
- d) (3 pts) The student presents reasonable evidence as in the likely approach analysis.
Extra credit could be awarded if the student
- a) Writes chemical equations for each reaction, particularly net ionic equations.
- b) reasons that although the evidence supports the identification of each solution from the limited list of three possibilities, additional tests might be performed to double-check the identification.
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