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Indicators: Identifying Acids and Bases

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Author: Janice VanCleave

Certain compounds react in acidic and/or basic solutions to form substances with specific colors. These compounds, or "indicators," can be used to identify the acidic or basic properties of household materials and foods.

In this project, you will determine the effect of acids and bases on homemade indicators. Carbon dioxide in exhaled breath will be tested for acidic properties, and you will determine whether exercise increases the carbon dioxide content in exhaled breath. You will also examine the pH (relative acidity) of a solution, and the effect of pH on indicators.

Getting Started

Purpose: To determine the effect that acids and bases have on the color of red cabbage extract.

Matsllllis

  • l-quart (l-liter) clear glass jar
  • red cabbage extract (see Appendix 5)
  • distilled water
  • marking pen
  • masking tape
  • 4 baby-food jars
  • white vinegar
  • 2 eyedroppers

Procedure

Caution: Handle the ammonia with care and work in a well-ventilated area. Ammonia is poisonous, and its fumes can damage skin and the mucous membranes of the nose, mouth, and eyes.

  1. Fill the quart (liter) jar half full with the red cabbage extract.
  2. Add distilled water to fill the jar so that the liquid is clear enough to see through but retains a definite purple color.
  3. With the marking pen, write "Cabbage Indicator" on a piece of masking tape and tape this label to the jar.
  4. Fill two baby-food jars one-fourth full with the diluted cabbage extract.
  5. Fill a third baby-food jar one-fourth full with vinegar, label it "Acid," and place one eyedropper in this jar.
  6. Fill a fourth baby-food jar one-fourth full with ammonia, label it "Base," and place the other eyedropper in this jar.
  7. Add drops of the acidic solution to one small jar of the cabbage indicator; swirl the jar to mix the solution after each addition. Continue until a definite color change is observed.
  8. Add drops of the basic solution to the other small jar of cabbage indicator; swirl to mix. Continue until a definite color change is observed (see Figure 11.1).

Results

The ammonia, a base, changes the cabbage extract to green. The vinegar, an acid, changes the cabbage extract to a pinkish red.

Why?

An indicator is a compound that changes color in the presence of an acid or a base. The color of any material is due to the chemical makeup of the substance that affects the light waves absorbed and reflected. Bases, solutions containing hydroxide ions (OH-), change the chemical structure of the cabbage so that it reflects more green light waves. Acids, solutions containing hydrogen ions (H+) change the structure of the cabbage so that it reflects more red light waves. These specific color changes allow red cabbage to be used to indicate the presence of an acid or a base.

Try New Approaches

  1. Use the instructions in Appendix 5 for preparing red cabbage extract to prepare extracts from other substances, such as blueberries, beets, grapes, or cherries. Repeat the experiment to determine the color changes of these extracts in the presence of an acid or a base. Not all indicators are affected by both acids and bases. You may discover an indicator that is specific (only changes color for an acid or a base).
  2.  Indicators Identifying Acids and Bases

  3. Turmeric is an indicator specific for the testing of a base. Prepare this indicator by adding 1/2 teaspoon (2.5 ml) of turmeric powder to 1/2 cup (125 ml) of rubbing alcohol. Repeat the original experiment to determine the color change in the presence of a base and the effect, if any, of an acid on the indicator.
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