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Hands-on Activity: Acid (and Base) Rainbows
Contributed by: Integrated Teaching and Learning Program, College of Engineering, University of Colorado at Boulder

Summary

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.

Engineering Connection

Relating science and/or math concept(s) to engineering

Measuring pH is fundamental science that is used by many engineers: Environmental and chemical engineers examine pollutant substances to find out whether they are acid or base to know what kind of reactions they cause. Electrical engineers design batteries. Chemical engineers design everything from pharmaceuticals to soap to glue to bubble gum. Civil engineers design water and waste treatment plants for towns and factories. Using the basic concept of acids and bases, engineers are able to create a lot of things that contribute to the health, happiness and safety of society.

Contents

  1. Learning Objectives
  2. Materials
  3. Introduction/Motivation
  4. Procedure
  5. Attachments
  6. Safety Issues
  7. Troubleshooting Tips
  8. Assessment
  9. Extensions
  10. Activity Scaling
  11. References

Grade Level: 6 (4-6) Group Size: 3
Time Required: 60 minutes
Activity Dependency :None
Expendable Cost Per Group
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Learning Objectives (Return to Contents)

After this activity, students should be able to:
  • 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.

Materials List (Return to Contents)

Parts 1 and 2

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.

Part 1 (only)

Each group needs:

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)
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.

Introduction/Motivation (Return to Contents)

All substances are divided into three categories — acids, bases and neutral substances. Acids and bases are some of the most important substances on Earth. There are strong and weak acids and bases, and their strengths are described by the pH scale.
Let's use the concept of money to communicate an understanding of acid/base strength. Assign a neutral pH (7) the value of 1 cent. A pH of 6 (or 8) is ten times stronger, so it is worth 10 cents. A pH of 5 (or 9) is ten times stronger than that, so it is worth 100 cents (or $1). A pH of 4 (or 10) is 10 times stronger than that, so it is worth $10.
When environmental engineers investigate a pollutant, they must find out whether the substance is an acid or base in order to know what kind of reactions it causes. Acid rain is an environmental problem that concerns many environmental and chemical engineers. The effects of acid rain include damage to the limestone and marble in statues and buildings; weakening of the exposed metal on bridges and cars; damage to bodies of water, wildlife, plants, forests and crops; and the contamination of the drinking water supply.
One way that we can help prevent acid rain is by burning less fossil fuel. We can also make laws that prevent large factories from burning fossil fuels or that require them to limit (minimize) their pollutant output. Engineers have developed many useful technologies for this purpose, but the companies must adhere to the laws. For example, emissions from cars have been reduced because cars now have catalytic converters that remove the poisonous gases from exhaust fumes.
Unfortunately, as we studied in the Air Pollution unit, Lesson 5, air and wind can move pollutants great distances. In fact, pollutants that contribute to acid rain may be carried hundreds of miles by wind before being deposited on the Earth. Because of this, it is sometimes difficult to determine the specific sources of these acid rain pollutants. For example, Canada has an acid rain problem because of manufacturing in the mid-western states of the U.S. Sulfur dioxides are produced in the industries in Ohio, Illinois and Pennsylvania, and are carried over the land by the weather patterns. The acids then combine with rain over Canada and the Adirondack mountains in New York, making those lakes lifeless.

Background: Identifying Acids and Basis

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.

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.
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.
Photograph of the ingredients to complete this experiment including paper cups, oil, milk, OJ, vinegar, juice and pH paper.
Figure 1. Activity supplies.

With the Students: Part 1 – Using pH Paper

  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.
A photograph shows a pH indicator that has turned red from a lemon and a pH indicator that has turned blue from milk.
Figure 2. Using pH paper. The lemon juice is an acid; the milk is a base.

With the Students: Part 2 – Using Cabbage Juice

  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.

Safety Issues (Return to Contents)

  • 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.

Troubleshooting Tips (Return to Contents)

You may want students to use calculators when calculating averages.

Pre-Activity Assessment

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

Activity Embedded Assessment

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

Post-Activity Assessment

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.

Activity Extensions (Return to Contents)

What happens to pH when you mix acids and bases? Students can add baking soda into each cup containing an acid. Measure how much baking soda must be added to return the cabbage juice to the original color. Make sure they keep one cup with only cabbage juice, as a control. The stronger the acid, the more baking soda must be added to return to the original color.
Determine the proportions of vinegar and baking soda necessary to create a neutral substance.
Use electronic pH probes to obtain more accurate pH information.
Test different antacids to see which type is the most effective.
Explore the damaging effects of acid on the environment.

Activity Scaling (Return to Contents)

  • 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

Contributors

Amy Kolenbrander, Sharon Perez, Janet Yowell, Natalie Mach, Gwendolyn Frank, Malinda Schaefer Zarske, Denise Carlson

Copyright

© 2004 by Regents of the University of Colorado.
The contents of this digital library curriculum were developed under a grant from the Fund for the Improvement of Postsecondary Education (FIPSE), U.S. Department of Education and National Science Foundation GK-12 grant no. 0226322. However, these contents do not necessarily represent the policies of the Department of Education or National Science Foundation, and you should not assume endorsement by the federal government.

Supporting Program (Return to Contents)

Integrated Teaching and Learning Program, College of Engineering, University of Colorado at Boulder

Last Modified: April 23, 2014
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