Lesson 18:
Natural Indicators
Concepts conveyed | Materials | Procedure | Benefits | Resource
Concepts conveyed:
The purpose of this demonstration is to show how indicators determine presence of acids and/or bases using dyes that can be found in materials that we use at home.
Materials:
- 1 sheet of golden rod (dark yellow/gold) paper
- ammonia-based window cleaner
- several red cabbage leaves
- zip-closing bag
- 3/4 cup of warm water vinegar
- laundry detergent powder
- other commonly used acids and bases (e.g. lemon juice, pickle juice, orange juice, cream of tarter, alum, etc.)
Procedure:
First, use the sheet of golden rod paper as an indicator by spraying it with ammonia-based window cleaner. The paper turns red. The dye in the paper is pH sensitive. In an acidic environment, the dye is yellow. In a basic environment, the dye turns red. Paper is normally made under acidic conditions; that is why the dye starts out as yellow. However, when the dye comes in contact with ammonia, which is a base, the dye turns red.
Next, make an indicator solution out of the red cabbage leaves by tearing them up, placing them in a zip-closing bag, adding 3/4 cup of warm water, and shaking the cabbage and water until the water turns dark blue (about 3 minutes). Use this indicator to test for acids and bases. It turns green in the presence of a base, and pink-tored in the presence of an acid. Try using vinegar (acid), laundry detergent powder (base), and/or ammonia (base). Also, use other products the students might use in their everyday activities.
If you have access to or know how to prepare solutions of known pH values, you can set up a series of test tubes with solutions of pH 1, 3, 5, 7, 9, 11, and 13. Place one or a few drops of cabbage extract in each and observe the rainbow of colors that result. You can use this series of colored test tubes for comparison to determine the pH of other substances around the house, such as those listed in the Materials section.
Benefits:
- Students will begin to recognize the utility of indicator chemistry in visual detection of acids and bases.
- This demonstration can be used to introduce the concept and reactions of acids and bases in the lecture.