Pre-Laboratory Discussion

Little pre-laboratory discussion is needed with this activity other than a safety review and ensuring that students understand the instructions, including use of the conductivity tester. Make clear to students that they will be classifying materials based on their reactions and insure that they understand in general what "classifying" involves. Do not use terminology such as acid, base, pH, etc.; the purpose of this activity is to encourage students to invent these concepts from relevant chemical experiences.

Teacher-Student Interaction

The effectiveness of this activity will depend on your interaction with small groups as they work within parts of the procedure and with the entire class as they prepare to move from Part I to Part II, and from Part II to Part III.

Part I
Move from group to group to ensure that each group sets up the reaction matrix as instructed. If you use test tubes rather than well plates, students may need to complete one reaction series and wash the tubes before they conduct the next nreaction series. In either case, be sure students record their observations.

To prepare students for Part II, point out that most solutions were 0.1 M concentration. Invite students to speculate on the results if similar tests were conducted with solutions that were (a) more concentrated or (b) less concentrated. Consider recording the predictions of several students on the chalkboard before proceeding to Part II.

Part II
Circulate to ensure that students correctly complete the dilution series and record their observations.

When students have completed their observations, check on whether they need help in the dilution calculations.

When students have answered Data Analysis Question 2 within their group, have them share their suggestions. When the gradation of colors associated with more and less acidity and less and more basicity is recognized by students, inform them that this is one way chemists identify the property called pH. Tell them that they will develop their understanding of pH as their acid-base study progresses. Universal indicator will continue to serve as a useful way to identify the pH of materials.

Point out that the idea of using universal indicator to determine pH will be used in the Part III of the activity. If students are to complete additional tests (in addition to universal indicator) provide suitable instructions.

Part III
As you initially circulate among students, ensure that they use only small samples of household materials and that they do not mix the materials with each other. Encourage groups to share household materials if only limited samples are on hand.

Have students share and compare information. Encourage them to use language such as "has the same color as 0.001 M NaOH" as well as "is an acid" or "is a base".

Both items in Part III Implications and Applications are open-ended; encourage students to evaluate their colleagues' answers. In dealing with the second question, encourage students to relate their hypotheses to their original observations and correlate answers with the pH chart found in Tips for the Teacher (item 8).

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