Abstract
This experiment is for all the kids out there who love boiled cabbage! You say you do not like cabbage? Well maybe you will like this amazing color-changing liquid you can make with cabbage. Which solutions around your house can make the cabbage juice change color? Find out while you learn about acids and bases and how to test for them.Summary
Objective
Make your own pH indicator and use it to test the pH of various household solutions.Introduction
A solution is a mixture of a soluble chemical dissolved in water. Think about the difference between salt water and tap water. The salt in the salt water has dissolved and the solution looks clear, but the salt is still there and will taste salty if you taste it. Because solutions are made with water, which is made of hydrogen and water, the hydrogen in the water can make a solution into an acid or a base.
You might think about an acid as something that an evil villain uses to trap a super hero, but actually some very common household solutions are acids. Acids are solutions that will donate hydrogen ions in a solution, and usually taste sour. Some common acids are citrus fruit juices and household vinegar. Bases are solutions that accept hydrogen ions in solution, and usually feel slippery. Bases have many practical uses. "Antacids" like TUMS or Rolaids are used to reduce the acidity in your stomach. Other bases make useful household cleaning products.
How do you tell if something is an acid or a base? You use a chemical called an indicator, which changes in color depending on whether a solution is acidic or basic. (Specifically, an indicator works by responding to the levels of hydrogen ions in a solution.) There are many different types of indicators, some are liquids and some are concentrated on little strips of "litmus" paper. Indicators can be extracted from many different sources, including the pigment of many plants.
Red cabbage contains an indicator pigment molecule called flavin, which is one type of molecule called an anthocyanin. This water-soluble pigment is also found in apple skin, red onion skin, plums, poppies, blueberries, cornflowers, and grapes. Very acidic solutions will turn anthocyanin a red color. Neutral solutions result in a purplish color. Basic solutions make a greenish-yellow or yellow color. For some examples, see Figure 1, below.
Figure 1. This picture shows some of the different colors that red cabbage juice can become. From left to right, the solutions shown range from very acidic (red) to very basic (yellow).
Because red cabbage has this indicator pigment, it is possible to determine the pH of a solution based on the color it turns the red cabbage juice. The pH of a solution is a numerical measure of how basic or acidic it is. A solution with a pH between 5 and 7 is neutral, 8 or higher is a base, and 4 or lower is an acid. For more detailed information, consult the Science Buddies guide to Acids, Bases, & the pH Scale.
Figure 2. Science Buddies board member Courtney Corda, her son, and another mother-daughter team demonstrate how to put cabbage to work as a pH indicator.
In this science experiment, you can extract your own cabbage juice indicator and use it to test the pH of different solutions around your home. You might be surprised to find out what common items around your house are acids or bases.
Terms and Concepts
- Solution
- Chemical
- Acid
- Base
- Indicator
- Pigment
- pH
Bibliography
This web site at Chemistry 4 Kids has a great tutorial on chemistry, matter properties and mixtures. Go check it out:
- Rader, Andrew. (2005). Matter is the Stuff Around You. Andrew Rader Studios. Retrieved December 13, 2005.
- Cobb, Vikki, 1972. Science Experiments You Can Eat. Harper Collins, New York, NY. Retrieved June 30, 2020.
Materials and Equipment
- A small red cabbage
- Boiling pot of water
- Strainer
- Small white Dixie cups (one for each household item you want to test the pH of)
- Medicine dropper
- Large bowls or pots (2)
- Lab notebook
- A series of household items to test the pH of:
- Fruit juice: lemon, lime, orange, apple
- Soda pop (dark sodas might be tricky to see)
- Vinegar
- Baking soda solution
- Cleaning products. Note: Always use caution when handling cleaning products.
- Anything you want!
Experimental Procedure
- Grate a small red cabbage and place the pieces into a large bowl or pot, as shown in Figure 3, below.
Figure 3. Grated red cabbage in a pot.
- Pour boiling water into the bowl to just cover the cabbage. Use caution when handling the boiling water.
- Leave the cabbage mixture steeping, stirring occasionally, until the liquid is room temperature. This may take at least half an hour. The liquid should be reddish purple in color, as shown in Figure 4, below.
Figure 4. While steeping in the water, the liquid in the pot should be reddish purple in color.
- Place a strainer over a second large bowl or pot and pour the mixture through the strainer to remove the cabbage pulp, as shown in Figure 6, below. Press down on the pulp in the strainer, such as by using a large spoon, to squeeze more liquid out of the pulp.
Figure 6. Cabbage pulp being removed from the mixture using a strainer.
- In the bowl, you should now have a clear liquid that will either be purple or blue in color, as shown in Figure 6, below. (It should look darker after the pulp is removed.) This will be your indicator solution.
Figure 6. This shows what the indicator solution can look like in a clear glass. (Note that you will be using the solution in white Dixie cups.)
- The color of the liquid will change depending upon the pH. Use Table 1, below, to figure out the pH of the liquid by observing the color.
| pH | Color |
|---|---|
| 2 | Red |
| 4 | Purple |
| 6 | Violet |
| 8 | Blue |
| 10 | Blue-green |
| 12 | Greenish-yellow |
- Set aside your indicator solution. You will use it as your "stock" solution for your experiments.
- Next you will test various household solutions with your indicator. Use a separate Dixie cup for each solution you want to test because you do not want to mix chemicals that do not go well together or contaminate your results.
- Fill about half of the Dixie cup with your cabbage indicator solution. You can use less indicator solution for each cup if you do not have a lot of indicator solution.
- Add drops of a liquid you want to test until you see the solution change in color. Gently swirl the cup as you add the drops, being careful not to spill the solution.
- Record the pH and a description of the each solution in a data table in your lab notebook like Table 2 below.
- Analyze your results. How does the pH of the different household items you tested compare to each other? Are you surprised by any of your results?
| Item | Color | pH |
|---|---|---|
| Lemon Juice | ||
| 7-UP | ||
| Add liquids to the table as you test them |
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Variations
- There are other plants that can be used to make pH indicators as well: red onion, apple skins, blueberries, grape skins, and plums. Experiment with different sources of pigment, to see which produce the best indicators.
- Can you use your pH indicator to conduct other useful experiments? What about testing the acidity of rain on a smoggy day? Or the usefulness of various antacid brands in reducing acidity of a solution?
- You can use an indicator to write secret messages. Just use full strength lemon juice to write an invisible message on paper and let the message dry. To reveal the message, paint indicator over the paper with a paint brush.
- You can actually buy pH strips that tell you the pH of a solution. How accurate is the pH indicator you made using cabbage compared to store-bought pH strips? You can purchase pH strips from aquarium stores or an online vendor such as Amazon.com.
Careers
If you like this project, you might enjoy exploring these related careers:
Related Links
- Science Fair Project Guide
- Other Ideas Like This
- Chemistry Project Ideas
- My Favorites
- Chemistry Safety Guide
- Acids, Bases, & the pH Scale
