To devise a scientific investigation to detect acids and bases in common materials.
In this activity, students will prepare a test solution whose color changes when an acid or a base is added; determine whether various household substances are acids or bases and look for patterns in the results; determine how their test solution compares to commercial acid-base testers; and search for other test solutions.
These activities can be a phenomenological introduction to acids and bases, an important concept for the study of chemistry. One of the properties of acids and bases is that they change the color of certain dyes. If these dyes are used to test a substance, the appearance of a particular color can be taken as an indication of the presence of an acid or base in the substance tested.
At this level, students need to become more systematic and sophisticated in conducting their investigations, some of which may last for weeks or more. That means closing in on an understanding of what constitutes a good experiment. The concept of controlling variables is straightforward, but achieving it in practice is difficult. Students can make some headway, however, by participating in enough experimental investigations (not to the exclusion, of course, of other kinds of investigations) and explicitly discussing how explanation relates to experimental design.
Have students discuss what they know about acids and bases, including giving examples of acidic and basic substances. Have the class agree on a couple of substances they believe are acidic (like vinegar or hydrochloric acid) as well as basic (like ammonia or lye). Generate a list of these substances and tell students they'll be testing some of these substances to see if they are indeed acids or bases.
Testing with Red Cabbage Juice
This part of the lesson introduces students to the phenomena in a guided manner, using the substances identified by students as acids or bases in the Motivation section. Have students work in cooperative groups to run color tests in small containers with white interiors (like small paper cups). Have them follow the procedure below.
- Add one tablespoon (15 ml) of cabbage juice to each of two cups. Describe the color of the juice.
- Keep one of the cups as a control (don't add anything to it). To the other cup, add a few drops of liquid acid or base or about one-eighth teaspoon of solid acid or base.
- Swirl to mix the test solution and acid or base. Immediately observe the color and describe it. (The solutions will be in the red range with added acid, and in the green to blue range with added base. The colors in base are particularly "fragile" and change from one hue to another in a few minutes.)
Questions to ask:
- If the cabbage juice is diluted with water, are the results the same? Are the colors easier or harder to distinguish and describe?
- What are the results if a more concentrated juice is made and tested?
After students have discussed their results, they can read an explanation of what is happening at Lose the Indicator Blues!, found on the WonderNet website.
Where are the acids and bases in your household?
Give students cabbage juice and have them test products from their homes. Students should consider a range of things from the kitchen, bathroom, and laundry. What can be tested is limited only by the need to use substances that are not too highly colored, since the colors may interfere with the tests. An example would be soft drinks; clear or lightly colored ones can be tested, but very dark colas probably won't give interpretable colors. In addition to just testing a lot of household substances, students could investigate whether solids mixed or ground with a small amount of water before testing give a different result than the solid alone.
Each student should write out the steps needed to carry out their investigations. In doing their investigation, they should carry out the steps and record their observations/data.
The activity must be brought together with a full class discussion of the results, including looking for patterns in the results. For example, some classes of substances may be mainly acidic or basic. Have students try to think of reasons for the observed patterns.
How do other acid-base tests compare to yours?
Begin by comparing student investigations using the pH Panel activity from the Miami Museum of Science. This includes substances such as baking soda, aspirin, lava soap, borax, and lemon juice. After students have explored acids and bases using the activities above, they are ready to extend their knowledge with further investigations.
The kinds of questions to investigate include:
- How do the results with cabbage juice compare to the results (for the same samples) from a commercial acid-base test kit for aquariums or soils?
- Which test can tell the difference(s) among the widest range of samples?
- What other kinds of highly-colored solutions (like fruit juices, for example) can be used to test for acids and bases?
- Can you make more convenient acid-base testers by soaking filter paper in the juice and using the dried paper as a tester?
These do not exhaust the questions students might want to investigate, but each student team ought to try at least one of these three, since the results will help the class gain a better understanding of how acids and bases work.
Each team should clearly define their investigation and procedures and document their work. This documentation should include a clear reporting of their data and an explanation of how they used controls in their investigation. Student groups should report their findings to the class.
Assessment of student work can be based on the quality of the data they gather to answer the question(s) investigated; the use of appropriate controls; and explanations that are consistent with experimental results and principles the students are expected to know and be able to use.
In their class presentations, students need to explicitly explain their experimental designs and discuss how they relate to their conclusions or explanations regarding acids and bases.
Don't hold your breath; see what's in it:
Students can work in cooperative groups, but each should get an opportunity to test her/his breath to determine its effect on the color of the cabbage juice. The most effective way to do the test is in test tubes about one-third full of the juice. One tube should be reserved as a control for color comparison and the student should blow gently and steadily through a straw into another. Changes take some time to become apparent, because the gases don't dissolve rapidly in the water.
After observations have been made and recorded, the class should discuss them and try to interpret them in terms of what they know about the gases in their exhaled breath (about 80% nitrogen, 16% oxygen, and 4% carbon dioxide). How might the interpretation(s) be tested? Are there ways to make and test the gas(es) that are thought to be responsible?