Science 10: Acids and Bases Lab

Science 10: Acids and Bases Lab Part 1 Introduction Refer to class notes on acids, bases and indicators.

Purpose The purpose of this part of the activity is to gain experience using various materials and equipment to test pH and to rank their usefulness. Then four unknown substances will be identified based on their pH.

Materials See Table1.

Apparatus Labeled spot plate Glass stirring rod Red and blue litmus paper pH paper (range 1-13) pH meter

Procedure Note: litmus paper should be cut into thirds to avoid waste.

1. Obtain a small amount of each substance into the wells of your labeled spot plate.

2. Prepare your litmus and pH paper. You should have a total of 14 pieces of each type of test paper.

3. Use your glass stirring rod to dab a drop of water on the test paper (dab, test, dab, test, dab, test). Note any observations in Table 1 or Table 2.

4. Rinse the stirring rod.

5. Repeat for all other substances.

6. Pay attention to the demonstration on phenolpthalein and record results in Table 1. Science 10: Acids and Bases Lab Table 1. pH results for known substances.

Substance Guess: Acid, Red Litmus Blue Litmus pH paper Acid, Base, or Neutral Phenolpthalein Neutral, or Base? Water

Lemon Juice

Club Soda

Bleach

Vinegar

Baking Soda

Ketchup

Dish Detergent

Table 2. pH results for unknown substances. Unknown Red Litmus Blue Litmus pH paper Substance 1 2 3 4 Science 10: Acids and Bases Lab Part 2

Introduction Red cabbage contains a pigment molecule called flavin (an anthocyanin). This water- soluble pigment is also found in apple skin, plums, poppies, cornflowers, and grapes. The color of the juice changes in response to changes in its hydrogen ion concentration. pH is the -log[H+]. Acids will donate hydrogen ions in an aqueous solution and have a low pH (pH < 7). Bases accept hydrogen ions and have a high pH (pH > 7). Therefore, it is possible to determine the pH of a solution based on the color it turns the anthocyanin pigments in red cabbage juice.

Purpose You will make a cabbage juice indicator and use your results from part 1 to create a color chart of pHs for your indicator. You will then use your chart and indicator to identify three unknowns.

Materials 2-3 leaves of red cabbage 5 aqueous solutions with the following properties: very acidic, slightly acidic, neutral, slightly basic, very basic

Apparatus 500 mL beaker 5 250 mL beakers Glass stirring rod

Procedure 1. Obtain 2-3 leaves of red cabbage. Chop the cabbage into small pieces. Place the cabbage in a 500 mL beaker and add boiling water to cover the cabbage. Allow at least ten minutes for the color to leach out of the cabbage. 2. Filter out the plant material to obtain a red-purple-bluish colored liquid. This liquid is at about pH 7. (The exact color you get depends on the pH of the water.) 3. Pour about 50 mL of your red cabbage indicator into each 250 mL beaker. 4. Add a small amount of each acid/base to each beaker. Stir. Observe and record color changes for each in Table 3. 5. Create a color pH chart for your cabbage juice indicator. Science 10: Acids and Bases Lab

Table 3. Qualitative results for five known acids’ and bases’ reaction with cabbage juice indicator.

Follow-up questions (staple to the back of your lab report)

1. Name the acids and name the bases that you discovered during this activity. Rank them from most acidic to most basic. (2 pts) 2. Why is it important to use both blue and red litmus paper to determine pH? (2 pts) 3. Did all your acids turn the indicators the exact same color? Why or why not? (2 pts) 4. Did all the bases turn the indicators the exact same color? Why or why not? (2 pts) 5. Were there any surprises in your data? Explain. (2 pts) 6. Which indicator do you think is the most reliable? Why? (2 pts) 7. What is the difference between strength and concentration in an acid or a base? Explain using examples. (2 pts) 8. Research the use of red cabbage in a historical/cultural context. Find at least three facts. State your source (is it reliable?). (4 pts)