Introduction

What if you could take a single liquid, and change it into a rainbow of colors without using food dye. You can! In this activity, you will use red cabbage to make what is called an indicator solution. Indicator solutions can change colors depending on what you add to them. In this case, adding something acidic (like lemon juice) will change it to one color while adding something basic (like bleach) will change it to another. Try and see how many colors you can make using different household acids and bases.

Time: 45 minutes to 1 hour

Key Concepts: chemical reaction, acids, bases, pH indicator

 

  • A small red cabbage
  • Lemon or lime juice
  • Vinegar
  • Optional: Other foods to test, such as clear soda pop, baking soda solution, egg whites, tomatoes, cottage cheese
  • Small white paper cups, drinking glasses, or small white dish (at least 3)
  • Grater
  • Strainer
  • Optional: Large spoon
  • A boiling pot of water
  • Large bowls or pots (2)
  • Bleach cleaning product

 

 

Prep Work

  1. Children should wear goggles or other protective eyewear and adults should supervise and use caution when handling bleach because it can irritate eyes and skin.

Procedure

  1. Grate a small red cabbage. Put the grated cabbage into a large bowl or pot.
  2. Boil a pot of water. Use caution when handling the boiling water. Pour the boiling water into the bowl with the cabbage pulp until the water just covers the cabbage.
  3. 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.
  4. Place a strainer over another large bowl or pot and pour the cabbage mixture through the strainer to remove the cabbage pulp. Press down on the pulp in the strainer, such as by using a large spoon, to squeeze more liquid out of the pulp.
  5. In the bowl, you should now have a clear liquid that will either be purple or blue in color. (It should look darker after the pulp is removed.) This will be your indicator solution.
  6. Fill a small white paper cup, drinking glass, or small white dish with 1 tablespoon (tbsp.) of your cabbage indicator solution. What is the color of your indicator solution?
  7. Add drops of lemon or lime juice to the indicator solution until you see the solution change in color. Gently swirl the solution and make sure the color stays the same. Question: What colour did the solution become?
  8. The color of the solution will change depending upon how acidic or basic it is. This is based on the pH of the solution. pH is a numerical measure of how acidic or basic something 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. The solution will be the following colors based on its pH: Red indicates pH 2; Purple indicates pH 4; Violet indicates pH 6; Blue indicates pH 8; Blue-green indicates pH 10; Greenish-yellow indicates pH 12. In summary, acidic solutions should be red or purple in color, neutral solutions should be violet, and basic solutions should be blue, blue-green, or greenish-yellow in color.
  9. Based on its color, what is the pH of the lemon or lime juice solution?
  10. In another small white paper cup, add 1 tbsp. of your original cabbage indicator solution. Add drops of vinegar until you see the solution change color. Question: What color did the vinegar solution become? What is the pH of the solution? Is it an acid or a base?
  11. In a third small white paper cup, add 1 tbsp. of your original cabbage indicator solution. Handling it with caution, add drops of the bleach cleaning product until you see the solution change color. Question: What color did the bleach solution become, and what does this indicate about its pH? Is it an acid or a base?
  12. If you want to test the pH of other foods, again add 1 tbsp. of your original cabbage indicator solution to a small white paper cup and add drops of the food until you see the solution change color. If the food is not in liquid form, crush it or dissolve it in a small amount of water before adding it to the indicator solution. What color did the solution become, and what does this indicate about its pH, and whether it is an acid or a base?

 

Cleanup

Dilute the bleach solution with water before pouring it down a drain.

 

What Happened?

A solution with a pH between 5 and 7 is neutral, 8 or higher is a base, and 4 or lower is an acid. Lime juice, lemon juice, and vinegar are acids, and so they should have turned the indicator solution a red or purple color. Bleach is a strong base and so it should have turned the indicator solution a greenish-yellow color.

 

Digging Deeper

Acids are solutions that lose hydrogen ions, “donating” them to the solution, and usually taste sour. Some very common household solutions are acids, such as citrus fruit juices and household vinegar. Bases are solutions that pull hydrogen ions out of solution and onto itself, “accepting” them, and usually feel slippery. Bases have many practical uses. For example, antacids like TUMS are used to reduce the acidity in your stomach. Other bases make useful household cleaning products.

To tell if something is an acid or a base, a chemical called an indicator is used. An indicator changes color when it encounters an acid or base. There are many different types of indicators, some that are liquids and others that are concentrated on little strips of “litmus” paper. Indicators can be extracted from many different sources, including the pigment of many plants. For example, red cabbages contain an indicator pigment molecule called flavin, which is a type of molecule called an anthocyanin. Very acidic solutions will turn anthocyanin a red color, while neutral solutions will make it purplish, and basic solutions will turn it greenish-yellow. Consequently, the color an anthocyanin solution turns can be used to determine a solution’s pH (which is a numerical measure of how basic or acidic it is).

 

For Further Exploration

  • There are other plants that can be used to make pH indicators as well: red onion, apple skins, blueberries, grape skins, and plums. Which different sources of pigment produce the best indicators?
  • You can use an indicator solution 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 paintbrush.

 

Additional Resources

Links

Science Careers

Credits
Teisha Rowland, PhD, Science Buddies