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Indicators: Identifying Acids and Bases (page 2)

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Author: Janice VanCleave

Design Your Own Experiment

  1. Use the food extracts previously prepared (red cabbage, blueberries, beets, grapes, or cherries) to test the acidic or basic properties of household materials, such as liquid and powder cleansers, cream of tartar, fruit juices, baking powder, and baking soda. Make solutions of the dry materials by adding 1 teaspoon (5 ml) of powder to 1/4 cup (63 ml) of distilled water. For each test, fill a small baby food jar onefourth full with the food extract and add 1 teaspoon (5 ml) of testing solution. Record the color and determine whether the solution has acidic or basic properties. Prepare a data table such as the one shown on the next page.
  2. Indicator testing paper can be made by soaking coffee filters in different liquid indicators made from food extracts, such as the red cabbage extract or the turmeric and alcohol solution. Allow the papers to dry, then cut them into 1-×-1/4-inch (2.5-×-6.3-cm) strips and store them in a closed container. Test for acids and bases by dipping about 1/4 inch (0.63 cm) of each testing paper into the solution. Observe the resulting color of each paper. Allow the paper strips to dry and display them on a poster board (see Figure 11.2).
  3. Demonstrate that paper indicators can be used to test for the acidic or basic nature of gases, such as ammonia, that are very soluble in water. Wet a piece of turmeric paper with water. Hold the wet paper above, but not touching, the mouth of an open bottle of household ammonia. Prepare and display drawings of the results. The ammonia gas dissolves in the water, and the product of this combination causes the turmeric to change colors. Write a chemical equation showing the combination of ammonia and water. Identify the product as an acid or a base and use the results of the turmeric paper to confirm this identification.
  4.  
    1. Carbon dioxide gas in your exhaled breath chemically reacts with water to produce an acid. Test for the presence of this acid by exhaling through a drinking straw into a glass soda bottle half filled with a solution of brom thymol blue indicator. (See Appendix 5) Hold a bottle of brom thymol blue indicator nearby to make the color comparison easier to see. A white background and good lighting will also make the comparison easier.
    2.  Indicators Identifying Acids and Bases

    3. Determine if exercise increases the concentration of carbon dioxide in exhaled breath by repeating the previous experiment after exercising. An increase in the rate of change and in the intensity of the color indicates an increase in carbon dioxide. Use photographs to represent the procedure and display an equation for the reaction along with them.
  5. Soil can have acidic or basic characteristics. Test the acidic/basic nature of a soil sample by combining 1 cup (250 ml) of soil with 2 cups (500 ml) of hot tap water. Stir well. Pour the mixture into a large funnel lined with filter paper. Fill a baby food jar half full with the filtrate (liquid that has passed through the filter paper). Fill the jar with red cabbage extract and stir. Use the color of the filtrate to determine its acidic or basic nature.

Get the Facts

  1. Each indicator has a particular concentration range over which the color change occurs. The measurement of the concentration of an acidic or a basic solution can be expressed as its pH; thus, indicators are affected by solutions with a specific pH. Find out more about indicators and the pH that affects them. What is the pH a measure of? Make a list of different indicators and their pH range.
  2. The pigment in red cabbage belongs to a class of compounds known as anthocyan ins. The red color of apples is also due to an anthocyanin pigment Find out more about the anthocyanin pigment. How is its color affected by the presence of an acid? A base? One source of information is "Solar Photography" in Janice VanCleave's A + Projects in Biology (New York: Wiley, 1993).
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