Focus question

How does red cabbage water indicate the presence of an acid or the presence of a base?

Essential materials

• Red cabbage
• Coffee filter or paper towel
• Container for water (at least 250 ml or 1/2 pint)
• Three transparent cups (about 100 ml or 3 ounces) or other similar containers
• Hot water* (e.g., from a faucet, heated in a microwave oven, etc.)
• Thermometer
• Vinegar
• Baking soda
• Safety goggles
• Tongs or fork
• Eyedropper or drinking straw
• Craft stick or toothpick
• Extra sticks of drinking straws for stirring

*If you do not have access to hot water in the classroom, you may wish to use an electric hot pot while carefully monitoring the temperature, or transport hot water to the classroom in a thermos or other suitable container.

Main ideas and background information

• An aqueous solution is water that contains a dissolved substance.
• When a substance dissolves in water, its molecules break apart or ionize. Each part of a molecule becomes a charged particle known as an ion.
• A hydrogen ion is a hydrogen atom whose electron has been removed.
• An acid is a substance that contributes hydrogen ions (H+) when dissolved in water.
• The pH scale is used to classify aqueous solutions, with a numerical range of 0.0 to 14.0 to indicate the concentration of hydrogen ions.
• A solution with a pH between 0.0 and 7.0 has a relatively high concentration of hydrogen ions and is called an acid. It usually has a sour taste. Citrus juices and vinegar are examples of acidic food products.
• A solution with a pH between 7.0 and 14.0 has a relatively low concentration of hydrogen ions and is called a base. It usually has a bitter taste. Lye soap and ammonia are examples of household products that are bases.
• Pure water is an example of a liquid with a pH of 7.0. Such a liquid has a neutral taste, neither sour nor bitter.
• A pH indicator is a substance that changes color according to the concentration of hydrogen ions. Typical indicators used by chemists include litmus paper and a solution known as phenolphthalein.
• Some food products also act as pH indicators. One of the more widely used products is red cabbage. When pieces of red cabbage are soaked in hot water, liquid with pigment from the cabbage mixes with the water, coloring it purple or bluish-purple. The color of this cabbage water is sensitive to pH changes. A high concentration of hydrogen ions (acid) shifts the color toward red or pink, while a low concentration of hydrogen ions (base) changes the color to light blue, green, or greenish yellow.
• For more information on pH, see Digging Deeper.
• The H in pH refers to the concentration of hydrogen ions present in the solution. The p comes from “potenz,” a German word referring to power or potential.
• In general, the color of this cabbage water corresponds approximately to the following pH ranges.
  

  Acid	0.0 to about 6.0	red or pink
  Near neutral	about 6.0 to about 8.0	purple or dark blue
  Base	about 8.0 to 14.0	light blue, green, or greenish-yellow

Procedural tips

• Students should tear the cabbage into pieces no larger than about 2.5 cm square (one inch square). 
• Have the students pack the cabbage pieces snugly into the container and tamp them down with their hands. The idea is to get as many cabbage pieces into the container as possible. This will maximize the amount of pigment that is transferred to the water.
• If you are using hot water from a faucet, be sure to wait until the running water is hot before filling the container of cabbage pieces.
• We recommend that the temperature of the water not exceed 50 degrees Celsius (about 125 degrees Fahrenheit). Check the temperature of the water with the thermometer as a safety precaution before making it available for student use, especially if you heat the water with a microwave oven or other appliance. You may wish to pour all of the hot water yourself.
• There is no need to disturb the cabbage pieces or the large container while waiting for the pigment to leach out into the water. However, doing so will not adversely affect the final water coloration.
• Students should wait until the water has cooled sufficiently before handling the large container.
• When removing the cabbage pieces from the large container, the idea is to remove as many as can be reached conveniently. One or two isolated cabbage pieces in the water will not be a problem.
• One student should hold the filter paper while another pours the cabbage water into the three small cups. Do not rush this process. The water will need a certain amount of time to move through the filter paper.
• Discuss with students the color of the cabbage water and reach consensus on its description. Is it purple? Is it blue? Perhaps record the color with a digital camera, if available.
• When adding vinegar and baking soda to the two small cups of cabbage water, stir the water for an even distribution. Compare any color changes to the color of the original water in the third cup.

Discussion

1. Why is the cabbage water colored?
   Answer: Pigment from the red cabbage leaves has gone into the water.

2. Why does the cabbage water change color when other substances are added to it?
   Answer: The pigment in the cabbage water is sensitive to changes in the concentration of hydrogen ions.

3. What happens to the color of the cabbage water when an acid or a base is added? 
   Answer: When an acid is added, the color shifts from purple to red or pink. When a base is added, the color goes toward light blue, green, or greenish-yellow.

Assessment

Are students able to describe how red cabbage water indicates the presence of an acid or the presence of a base?

Answer: The color of the cabbage water changes. Acids produce a red or pink color, while bases produce a blue, green, or greenish-yellow color.

Extensions and further investigations

• Have students make another batch of cabbage water and experiment with other household products, such as soap, detergent, and cooking oil.
• Set up an experiment to find out if other foods can be used to make effective indicator solutions. Beets, blueberries, and the petals of flowers such as pansies and violets might be good candidates.

Career connections

• Chemical research and engineering

• Coloration specialist

• Medical technician

• Water treatment plant operator

• Swimming pool maintenance person

• Aquarium manager

• Wetlands manager

• Agricultural engineer
  

Correlations with Standards

United States: This activity correlates with portions of NSES Content Standard A, Science as Inquiry, and Content Standard G, History and Nature of Science, Grades 5-8 and 9-12, and with the following additional standards:

Grades 5-8
Physical Science: B1

Grades 9-12
Physical Science: B3

Glossary/vocabulary

acid
aqueous solution
base
hydrogen ions
pH
pH indicator

Resource links

pH
Down to earth description of the pH scale.

Sambal's Science Web : indicators
Simplified information about red cabbage and pH indicators.

Wikipedia : PH indicator
Good overview of pH indicators with color chart and listing of various foods that can be used as well. Also has selection for several languages other than English.

Chem4kids.com : Acids and Bases are Everywhere
An excellent supplemental web site.

Miami Museum of Science : The pH Factor
An interesting site with interactive links.