Hands-on Activity: Acid (and Base) Rainbows
Educational Standards :
Learning Objectives (Return to Contents)
After this activity, students should be able to:
Materials List (Return to Contents)
Parts 1 and 2
Each group needs:
Part 1 (only)
Each group needs:
Part 2 (only)
Part 2: Preparation of red cabbage juice for the entire class to share (10 groups):
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.
Procedure (Return to Contents)
Background: Identifying Acids and Basis
Use the following information to help in the identification of acids and bases.
Characteristics of Acids
Characteristics of Bases
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
Cabbage Juice Preparation
With the Students: Part 1 – Using pH Paper
With the Students: Part 2 – Using Cabbage Juice
Attachments (Return to Contents)
Safety Issues (Return to Contents)
Troubleshooting Tips (Return to Contents)
You may want students to use calculators when calculating averages.
Assessment (Return to Contents)
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.
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)
References (Return to Contents)
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
ContributorsAmy 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