Chemistry Lab – Red Cabbage & Acid and Bases

Project 2

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Evaluation 32

Chemistry 2 (SCIH 032 062)

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Project 2- Acids, Bases, pH, and the Kitchen

As you have learned, the level of acidity in a solution can be measured by the pH of the solution. The pH of a solution is approximated using a base ten logarithm function of the concentration of the hydronium concentration (expressed as M) in the solution. The expression is:
pH = – log [H3O+].

From the expression, you can see that solutions with higher [H3O+] will have lower pH values. We say that those solutions are more acidic. When a solution has a pH = 7.0, the solution is described as neutral. You can also see that a change in value of one pH unit represents a ten-fold change in [H3O+].

A relatively convenient way to determine the pH of a solution has been developed based on color changes in specific compounds known as indicators. A typical indicator is a weak acid that has one color when it retains its acidic H+, but a different color after it loses its acidic H+.

In this experimental project, you will first obtain your own indicator by extracting a compound from the leaves of red cabbage. Then you will use this extract to measure the pH of several common substances. Finally, you will use the indicator color changes to study reactions between acids and bases. It is estimated that you will spend approximately sixty minutes on this project, which is worth a total of 100 points.

Be sure to follow all of these rules as you complete the project:

  • WEAR SAFETY GOGGLES WHILE PEFORMING ALL STEPS; ALLOW SOLUTIONS TO COOL BEFORE HANDLING.
  • Cabbage juice can be stored in a refrigerator overnight, but should be disposed of with water down the drain before mold growth begins.

Laboratory Materials List

The Projects in this course require special materials. Some of these materials are found in the Chemistry 2 Lab Kit and some will need to be provided by you, the student. Before doing a project, gather all the items you will need for that specific project and out them on a clear work space.

Student Will Supply Materials In Kit

strainer 1 well plate

approximately 1/3 of a head of fresh red cabbage safety googles

cooking container for boiling cabbage 2 droppers

measuring spoons

various household chemicals with acid/base properties. (A short, but not complete list is: vinegar, pickle juice, orange juice, clear soda, cream of tartar, antacid, ammonia-based cleaner, baking soda, etc.)

NOTE:

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Part A: Obtain Cabbage Juice Indicator

  • Carefully cut about 1/3 of a head of fresh red cabbage from the rest of the head of cabbage.
  • Carefully chop the cabbage into small pieces.
  • Boil the cabbage pieces in water for approximately ten minutes. Use enough water to cover the cabbage.
  • Strain the purplish juice into a glass container to cool. Discard the cabbage leaves.
  • NOTE: If you do not have red cabbage, you may substitute apple skins, plums, poppies, cornflowers, or grapes (all of which contain a pigment molecule called flavin). If you use a substitute, use at least two cups.

Part B: Calibrate the Cabbage Juice

The following table may be used to approximate the pH and color correlations for the cabbage juice indicator:

Color

red

reddish-purple

purple-violet

blue

bluish-green

green

pH

~2

~4

~6

~8

~10

>12

  • Obtain a small glass vial or small paper drinking cup and add about two medicine dropper “squirts” of cooled cabbage juice to the container.
  • Select a common kitchen solution and add two “squirts” of the solution to the same container. Note the color and estimate the pH of the solution.
  • Repeat steps 2 and 3 for each solution. Be sure to collect sufficient samples to obtain both acids and base results. Minimum of at least 4 base samples and 4 acid samples.
  • Solids may be used if only a small amount (less than 1/8 teaspoon dissolved in two squirts of water).

Calibrate the Cabbage Juice

Substance

Color of Solution

~pH

Acid or Base?

1.

   

2.

   

3.

   

4.

   

5.

   

6.

   

7.

   

8.

   

Part C: Neutralization Reactions

  • Step 1: In two separate clean, small containers (glass vial or small paper cup), place about ten drops of an ammonia cleaner in each container and two “squirts” of cabbage juice indicator in each container.
  • Step 2: Select two separate calibrated acid substances from your Part B Data Table. Try and select two that range in pH.
  • Step 3: Using your 1st container solution from Step 1 and a clean medicine dropper. Count the number of drops of your 1st calibrated acid required to change the color of the solution from green to purple. Summarize your results below in the table.
  • Step 4: Using your 2nd container solution from Step 1 and a clean medicine dropper. Count the number of drops of your 2nd calibrated acid required to change the color of the solution from green to purple. Summarize your results below in the table.
  • Step 5: Proceed to Part D Discussion Questions.

Neutralization Reactions

  • A comparison of two acid solutions neutralizing a base cabbage juice + ammonia solution.

Acid

pH

Number of Drops of Acid to Change Solution to Purple

1.

  

2.

  

Part D: Discussion Questions

Write your answers in the space provided after each question. Each question is worth twelve points.

1. Use an Internet or library source to determine the type of compound(s) found in the cabbage juice indicator that is actually changing color. Identify the compound(s) and provide information about their characteristics.

2. What other “natural” plant indicators can be isolated? Need several indicators & their sources.

3. What are the identity and pH of the two most acidic solutions that you measured? Provide units of measurements for each substance.

4. What are the identity and pH of the two most basic solutions that you measured? Provide units of measurements for each substance.

5. Suppose two different acid solutions have the same pH. Is it correct to say that the solutions have the same concentrations of [H3O+]? Is it correct to say that the two acid compounds making the two solutions are the same strength? Explain why for each question.

6. If a solution has a pH = 3, and another solution has a pH = 5, in terms of acidic strength, how many times weaker is the second solution? Show your calculations.

7. In the titration of ammonia solution with vinegar, and with another acid solution with a slightly higher pH value, how did the amounts needed for the neutralization of the ammonia (signaled by the change from green to purple in the solution containing the ammonia and cabbage juice) compare? How does this comparison match the differences in pH values? What is the basis of any correlation between starting pH values of the two acids and the amount required to neutralize the same amount of vinegar solution? Provide explanations for each of the three questions.

This project can be submitted electronically. Check the Project page under “My Work” in the UNHS online course management system or your enrollment information with your print materials for more detailed instructions.

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