Anticipated Student Results

All acids should cause each indicator to exhibit a characteristic color; the bases should cause a different color to appear. Water should cause the indicators to turn a color that is slightly or greatly different from the color produced by either acids or bases.

Magnesium metal will react with acids to produce gas bubbles, but should not react with bases. All acids and bases should be electrically conductive; water is not. Bases used in this activity should produce a precipitate (cloudiness) with calcium nitrate; acids should not.

If you are using universal indicator as recommended, the tested solutions should grade from red through yellow and green to blue and violet, following the visible light spectrum from most acidic (red) to most basic (violet).

Answers to Data Analysis

Part I

1. When analyzing data, student groups should compare classification schemes and then discuss any differences noted. After discussion, acids should be grouped together due to their common indicator colors, reactivity with magnesium, and failure to react with calcium nitrate. Bases should be grouped together based on indicator color, failure to react with magnesium, and reaction with calcium nitrate. Water should be identified as belonging to neither group. At this point, and not before this point, introduce the terms "Acid," "Base," and "Neutral." These terms, as used by chemists, refer to the properties (and others not yet encountered) just observed by the students.

2. The ions in solution are: HCl(aq) contains H⁺(aq) and Cl^-(aq), NaOH(aq) contains Na⁺(aq) and OH^-(aq), H2SO4(aq) contains H⁺(aq) and HSO4^-(aq) [remember HSO4^-(aq) is a weak acid] and some SO4^2-(aq), HNO3(aq) contains H⁺(aq) and NO3^-(aq), KOH(aq) contains K⁺(aq) and OH^-(aq), and Ba(OH)2(aq) contains Ba²⁺(aq) and OH^-(aq).

3. HCl(aq), H2SO4(aq), and HNO3(aq) are acids; NaOH(aq), KOH(aq), and Ba(OH)2(aq) are bases.
   

Part II

1. For the HCl solutions: Solution 1 is 0.1 M, 2 is 0.01 M, 3 is 0.001 M, 4 is 0.0001 M, 5 is 0.00001 M. For the NaOH solutions: Solution 5 is 0.00001 M, 4 is 0.0001 M, 3 is 0.001 M, 2 is 0.01 M, 1 is 0.1 M.

2. Follow the spectrum from red to violet as the solutions become less acidic and more basic.
   

Part III

1. The results depend on which household materials are tested. For general results see the pH chart in Tips for the Teacher (item 8).
   

Post-Lab Discussion

The suggested class development outlined above is intended to introduce the concepts of acid, base, pH, and possibly indicators from phenomena students directly observe in the laboratory. Thus the post-lab discussion can employ a form of "stand-and-deliver" strategy in which you call on students to define main concepts and illustrate definitions with examples from reactions just studied. Other students can be invited to evaluate or extend previous comments by their colleagues.

	TABLE OF CONTENTS	TOPIC OVERVIEW	CONCEPT/SKILLS DEVELOPMENT	LINKS/CONNECTIONS	EXTENSIONS