Leading the Activity
Discuss that observing chemical changes (color, in this case) can be used to classify the characteristics of substances. Tell the class that the juice from red cabbage will change color in the presence of acids and bases.
Have the students use the cabbage juice indicator to test several liquids which have
been previously tested with the wide range (0 14) pH test strips. Do not tell the
students which color changes in the cabbage juice indicate acids or bases. Allow
time and opportunity for the students to discover this for themselves by comparing their observations with previous test results.
Procedure
Preparing the Cabbage Juice Indicator
- The cabbage juice may be prepared at home or with students in the classroom. Cut the red cabbage into several pieces and place them in a pot with enough water to cover
the cabbage, about 1 2 quarts. Boil the water and cabbage for 10 15 minutes and let
it cool. Pour the contents through a strainer into a container and discard the cabbage
leaves. The cabbage juice should be stored in the refrigerator until needed and may
be frozen if it will be stored for several days.
Testing Liquids with the Cabbage Juice Indicator
- Combine equal amounts of cabbage juice and the liquid to be tested in a small, clear container. Observe the color change and record the results on a chart. Compare the observations with the results found using the indicator paper strips during the telecast.
The cabbage juice can be used to make a paper strip indicator. Cut strips of coffee
filter paper and soak them in the cabbage juice. After they are dry, use them as
you would use the commercial test strips included in the kit.
Range of Results
Boiling the red cabbage will yield a liquid which is purple to bluish in color. After
some use, the liquid will tend more toward blue than purple, but the reaction indicating acids and bases should still be accurate. Acids will turn the cabbage juice pink. Bases will turn the juice green. If the cabbage juice becomes contaminated, you can add a small amount of acid or base to return it to the bluish-purple neutral color--or simply boil a new batch of indicator juice.
The cabbage juice indicator may have an odor which students find unpleasant, particularly if you boil the cabbage in the classroom. Mention this beforehand to prepare the students. Of course, you can avoid the classroom problem by preparing the cabbage juice indicator at home.
Results should match the data collected with the commercial indicator papers. Encourage the students to test some of the same liquids and compare the results with both indicators.
Teacher Background Information
As mentioned earlier, a description of why the indicator changes color in the presence
of acids and bases is not important. Allow the students to use the cabbage juice
as a tool to test and classify materials as acidic, neutral, or basic.
For More Ideas and Information
Using red cabbage to prepare an acid/base indicator is described in many science resource books and activity guides. Janice VanCleave's Chemistry for Every Kid, published by John Wiley & Sons, Inc., 1989, describes using cabbage juice and turmeric to prepare paper indicator strips.
One of the most in-depth coverages of using cabbage juice to investigate acids and
bases is found in Of Cabbages and Chemistry, one of the Great Explorations in Math
and Science (GEMS) guides from the Lawrence Hall of Science at the University of
California at Berkeley. GEMS is an ongoing curriculum development project with a variety of excellent teacher guides. A descriptive brochure and ordering information is available.
Write or call:
GEMS
Lawrence Hall of Science
University of California at Berkeley 94720
(415) 642-7771
