Students are introduced to the differences between acids and bases and how to use indicators, such as pH paper and red cabbage juice, to distinguish between them. They learn why it is important for engineers to understand acids and bases.
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- Colorado: Math
- b. Solve multi-step word problems involving fractions, decimals and whole numbers (Grade 6)  ...show
- Colorado: Science
- d. Examine, evaluate, question, and ethically use information from a variety of sources and media to investigate how environmental conditions affect the survival of individual organisms (Grade 6)  ...show
- Common Core State Standards for Mathematics: Math
- International Technology and Engineering Educators Association: Technology
- F. Knowledge gained from other fields of study has a direct effect on the development of technological products and systems. (Grades 6 - 8)  ...show
- Next Generation Science Standards: Science
- Make observations and measurements to identify materials based on their properties. (Grade 5)  ...show
- Describe some general characteristics of acidic, basic and neutral solutions.
- Calculate and interpret the averages of pH trials.
- Explain why engineers measure the pH of substances with which they are working.
- Use their knowledge of pH to describe some effects of acid rain.
Parts 1 and 2
- 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:
- Vinegar (2 samples of about 2 tablespoons each)
- Lemon juice (about 2 tablespoons)
- Tomato or apple juice (pure) (about 2 tablespoons)
- Distilled water (2 samples of about 2 tablespoons)
- Salt water (2 tablespoons of this mix: 3 tablespoons salt in 1 cup distilled water)
- Household liquid bleach (about 2 tablespoons)
- Milk of Magnesia (about 2 tablespoons)
- Baking soda (about 2 tablespoons in 1 cup distilled water)
- Alka-Seltzer tablet
- Alka-Seltzer tablet
- Plus the specific items below, for Part 1 or 2.
Part 1 (only)
- 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
Part 2 (only)
Part 2: Preparation of red cabbage juice for the entire class to share (10 groups):
- 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)
Background: Identifying Acids and Basis
- Tastes sour.
- Turns litmus paper red.
- Turns cabbage juice red.
- Tastes bitter.
- Feels slimy.
- Turns litmus paper blue.
- Turns cabbage juice yellow, green or blue, depending upon the solution concentration.
- 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.
Before the Activity
- 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.
- Place small pieces of cabbage into a blender.
- Pour cold, distilled water into the blender and let it sit for one hour.
- Blend on high speed until thoroughly mixed.
- Strain the cabbage juice into a pitcher/beaker using a strainer (or a funnel and coffee filters).
- Place samples in small jars or containers, one for each student group.
With the Students: Part 1 – Using pH Paper
- Distribute 10 cups, 2 Alka-Seltzer tablets, pH paper and Measuring pH of Common Substances Data Sheets and Worksheets to each group.
- Ask students to record on their data sheets their predictions for the pH of each item.
- 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).
- 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.
- Have students compare their results to the items listed on the attached pH Values of Common Substances.
- Have students answer the questions on their Measuring pH of Common Substances Worksheet.
With the Students: Part 2 – Using Cabbage Juice
- Explain to the students that cabbage juice is a natural substance that can be used to test (indicate) for acids or bases.
- 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.
- Distribute a Cabbage Juice Rainbow Data Sheet and a container of cabbage juice to each group.
- 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.
- 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.
- Have students record their results on their data sheets.
- Discuss/compare results as a class.
- Measuring pH of Common Substances Data Sheet
- Measuring pH of Common Substances Data Sheet Answers
- Measuring pH of Common Substances Worksheet
- Measuring pH of Common Substances Worksheet Answers
- pH Values of Common Substances
- Red Cabbage Juice Indicator Color Chart
- Cabbage Juice Rainbow Worksheet
- Cabbage Juice Rainbow Worksheet Answers
- Check with students about allergies to food and other substances used in this activity.
- Be sure to explain that scientists and engineers never test substances by tasting them (even though we know that some of the things we are working with are edible)!
- Direct supervision is necessary when working with these materials since some may be irritating to the nose or skin.
Activity Embedded Assessment
- For lower grades, use fewer examples and test them as a class demonstration.
- For lower grades, the concept of "average" may be difficult, so consider eliminating this portion or come up with a class average for the pH of a given substance.
- For lower grades, have students make a bar graph detailing their results.
Acid Rain Lesson Plan (Grades 6-8). Updated December 18, 2003. National Park Service. Accessed August 16, 2004. (Activity adapted from this source.) http://www.nature.nps.gov/air/edu/Lessons/AcidRLessonPlan.cfm
Results of Adding Cabbage Juice to Solutions of Different pH Values. CSU Stanislaus Science Web, Turlock, CA. Accessed November 9, 2004. (Source of red cabbage indicator color chart attachment) http://wwwchem.csustan.edu/chem3070/images/cabbage.gif
Amy Kolenbrander, Sharon Perez, Janet Yowell, Natalie Mach, Gwendolyn Frank, Malinda Schaefer Zarske, Denise Carlson
© 2004 by Regents of the University of Colorado.
Integrated Teaching and Learning Program, College of Engineering, University of Colorado Boulder
Last modified: October 6, 2015