Each group needs:

• 10 clear 6-8-oz. cups or Dixie cups to hold test materials
• Masking tape and pen (for labeling cups)
• The following 10 test materials:

1. Vinegar (2 samples of about 2 tablespoons each)
2. Lemon juice (about 2 tablespoons)
3. Tomato or apple juice (pure) (about 2 tablespoons)
4. Distilled water (2 samples of about 2 tablespoons)
5. Salt water (2 tablespoons of this mix: 3 tablespoons salt in 1 cup distilled water)
6. Bleach (ammonia) (about 2 tablespoons)
7. Milk of Magnesia (about 2 tablespoons)
8. Baking soda (about 2 tablespoons in 1 cup distilled water)
9. Alka-Seltzer tablet
10. Alka-Seltzer tablet

• Plus the specific items below, for Part 1 or 2.

Each group needs:

• 30 1.5-inch strips of wide-range (0-14 pH) litmus paper and comparison chart. Students need to use the comparison chart included with the litmus container, so obtain enough dispensers for each group to have one. Purchase litmus paper from a chemistry supply company (such as Fisher) or a well-equipped hardware store.
• pH Values of Common Substances attachment (or as an overhead projector transparency for the entire class to view)
• Measuring pH of Common Substances Data Sheet, and Measuring pH of Common Substances Worksheet, one per student

• Copies of the Cabbage Juice Rainbow Data Sheet, one per student
• Copies of the Red Cabbage Indicator Color Chart (one per group, or one color overhead projector transparency for the entire class to view)
• Cabbage juice for the entire class to share (see materials list, below)

• 1 small red cabbage
• Cold, distilled water
• Blender (for teacher use only)
• Fine mesh strainer (or funnel and a few coffee filters)
• Large pitcher or beaker
• 10 small jars, beakers or containers (500 ml size, 1 per group)

Alternate Setup: To reduce cost and preparation time, make one sample of each solution and have the students rotate through the samples (instead of each group having each sample). Make numerous water and vinegar samples so that each group has a sample in which to put the Alka-Seltzer. Have each group be in charge of using cabbage juice to test just one of the samples and share data as a class.

Use the following information to help in the identification of acids and bases.

Characteristics of Acids

• Tastes sour.
• Turns litmus paper red.
• Turns cabbage juice red.

Characteristics of Bases

• Tastes bitter.
• Feels slimy.
• Turns litmus paper blue.
• Turns cabbage juice yellow, green or blue, depending upon the solution concentration.

Acid/Base Indicators

• Indicators change color with different pH values.
• Litmus paper is frequently used to investigate the pH of substances. The more basic a substance, the bluer the indicator turns. The more acid, the redder the paper turns.
• Cabbage juice can be used to make a rainbow of colors corresponding to various (approximate) pH values.

More about Acid-Base Indicators in Red Cabbage

Red cabbage contains two main types of plant dyes: anthocyanin and flavonol. Anthocyanin pigments are red in strongly acidic solution, blue in neutral and weakly basic solutions, and colorless in strongly basic solutions. Weakly acidic solutions contain some of the red form and some of the blue form and thus appear purple. Flavonol pigments are colorless in acidic and neutral solutions, and yellow in basic solutions. Weakly basic solutions thus contain both blue (anthocyanin) and yellow (flavonol) dyes, and appear to be green. The pH corresponding to various colors varies slightly with concentration, solvent, age and variety of cabbage. Most flowers and fruits contain anthocyanin as pigments.

• Prepare the cabbage juice indicator the morning of the experiment (see below for instructions).
• Gather materials (see Figure 1). If time permits, place samples of the 10 test substances into small cups.
• Make copies of the handouts.

Cabbage Juice Preparation

1. Place small pieces of cabbage into a blender.
2. Pour cold, distilled water into the blender and let it sit for one hour.
3. Blend on high speed until thoroughly mixed.
4. Strain the cabbage juice into a pitcher/beaker using a strainer (or a funnel and coffee filters).
5. Place samples in small jars or containers, one for each student group.

Figure 1. Activity supplies. click for copyright

1. Distribute 10 cups, 2 Alka-Seltzer tablets, pH paper and Measuring pH of Common Substances Data Sheets and Worksheets to each group.
2. Ask students to record on their data sheets their predictions for the pH of each item.
3. Demonstrate the use of pH paper. For example, ask each student to test the pH of the lemon juice, and show them how to use the comparison chart on the pH paper package to determine the pH. You may also want to demonstrate how to calculate the average pH (a sample calculation may be found at the bottom of their data sheets).
4. Ask the students to finish testing each of the 10 items (see Figure 2). Have them add one Alka-Seltzer tablet to one of the water samples and one of the vinegar samples. Ask the students to record all tests on their data sheets and establish whether each item is an acid or base.
5. Have students compare their results to the items listed on the attached pH Values of Common Substances.
6. Have students answer the questions on their Measuring pH of Common Substances Worksheet.

Figure 2. Using pH paper. The lemon juice is an acid; the milk is a base. click for copyright

1. Explain to the students that cabbage juice is a natural substance that can be used to test (indicate) for acids or bases.
2. Provide each group with a color printout, or show the class a color overhead projector transparency (Red Cabbage Indicator Color Chart attachment) of the range of cabbage juice color changes, corresponding to whether the solution is an acid or base.
3. Distribute a Cabbage Juice Rainbow Data Sheet and a container of cabbage juice to each group.
4. Ask students to make predictions about the expected color change for each substance and record this on their data sheets. They should guess fairly accurately if they were paying attention in Part 1.
5. Ask each group to add cabbage juice to each of the cups until they see a color change. If you like, you can have students measure how much cabbage juice they are adding.
6. Have students record their results on their data sheets.
7. Discuss/compare results as a class.

Predictions: Ask students to record their predictions for the pH of each item on their Measuring pH of Common Substances Data Sheet.

Data Sheets: Ask students to record all tests on the data sheets and establish whether each item is an acid or base.

Discussion: Ask students what they have learned. Ask if there were any surprises. Discuss student answers to questions on the Measuring pH of Common Substances Worksheet.

Human pH Scale: On separate pieces of paper, write the names of the different substances you tested. Ask for volunteers from the class to come up to the front of the room, and give each one of the pieces of paper. Have all volunteers read what is written on their papers one at a time. Have the rest of the class label them as acid, base or neutral by voting. Have students "terms" stand in order on the pH scale. At the end, review the concepts learned.