CHEMLAB Household Acids and Bases

Indicators often are used to determine the approximate pH of solutions. In this ChemLab, you will make an indicator from red cabbage and use the indicator to determine the approximate pH values of various household liquids. The cabbage juice indicator contains a molecule, anthocyanin, that accounts for the color changes.

PROBLEM

What are the approximate pH values of various household liquids?

OBJECTIVES

• Measure and compare the pH values for various household liquids.

• Compare the functions of the liquids to their chemical makeup.

HYPOTHESIS

MATERIALS

red cabbage

toothpicks

hot plate

beaker tongs

100-mL beakers (2)

distilled water

microtip pipets (9)

96-well microplate

piece of white paper

100-mL graduated cylinder

solutions of: eyewash, lemon juice, white vinegar, table salt, soap, baking soda, borax, drain cleaner

SAFETY PRECAUTIONS

Use beaker tongs to handle hot beakers.

Wear an apron and goggles. Some of the solutions to be tested are caustic, especially the drain cleaner. Avoid all contact with skin and eyes. If contact occurs, immediately wash with large amounts of water and notify the teacher.

PROCEDURE

1. Tear a red cabbage leaf into small pieces, and layer the pieces in a 100-mL beaker to a depth of about 2 cm. Add about 30 mL of distilled water.

2. Set the beaker on a hot plate, and heat until the water has boiled and become a deep purple color. Remove the beaker from the hot plate using beaker tongs, and allow it to cool. Pour off the cabbage juice indicator liquid into a clean beaker.

3. Set a clean microplate on a piece of white paper. Use the pipets to add 5 drops of eyewash to well H1, lemon juice to H2, white vinegar to H3, and solutions of table salt to H4, soap to H5, baking soda to H6, borax to H7, and drain cleaner to H8. Use a clean pipet for each solution.

4. Draw the cabbage juice indicator solution into a clean pipet, and add 5 drops to each of the solutions in wells H1-H8. Stir the solution in each well with a clean toothpick.

5. Looking down through the wells, note and record the color of each solution in the data table. Using the color chart on page 504 of your textbook, record in the data table the approximate pH of each of the solutions.

ANALYZE AND CONCLUDE

1. Interpreting Data Are food items such as lemon juice or vinegar acidic or basic? These solutions are either tart or sour, so what ion probably accounts for this characteristic?

2. Interpreting Data Were the cleaning solutions acidic or basic? What ion is probably involved in the cleaning process?

3. Observing and Inferring How can you account for the great pH difference between lemon juice (citric acid solution) and eyewash (boric acid solution)?

4. Using Variables, Constants, and Controls Suppose that, in addition to the solutions, you tested a well containing pure distilled water. What purpose would this test serve?

APPLY AND ASSESS

1. Would your indicator work well to determine the pH of ketchup? Explain.

2. You may have noted that some shampoos are described as pH-balanced. What do the manufacturers mean by this phrase? Why would they do this to a soap or detergent?

3. Hypothesize about how other solutions at home would react with the cabbage juice indicator. Explain your predictions.