TEACHER:

Jan E. Sorensen, biology
Fairview High School
Boulder, Colorado

AGES:

Six to 12

PROJECT:

Use cabbage juice to test the pH of household substances

GOAL:

To establish an understanding of acids, bases and pH



Jan Sorensen loves the hands-on nature of biology and the cabbage juice pH indicator project is a favorite. "Kids think that laboratory science is going to be dramatic, with things blowing up all the time," she says. "By using an everyday item like the purple cabbage, they learn that chemistry is going on around us all the time."

Scientists use pH measurements to determine the concentration of hydrogen ions in a particular substance which tells them whether the subject they're testing is an acid or a base. The higher the concentration of hydrogen ions, the more acidic it is. The pH scale ranges from zero (most acidic) to 14 (most basic) with seven as the neutral point.

Most things in the world can be tested for pH and many of the living things around us respond in vivid ways to pH changes. You might tell your kids that some flowering plants will produce different-colored blooms depending on whether the soil they grow in is acidic or basic.

MATERIALS
• 1/2 head of purple cabbage
• Isopropyl alcohol (optional)
• Large glass jar
• Seven small jars
• Drinking glass
• Distilled water
• Eyedropper
• Vinegar or lemon juice
• Marker
• Bowl
• Baking soda
• Any products you'd like to test--shampoo, soap, soda, baking powder

INSTRUCTIONS
1. Chop up the cabbage and boil it for 15 minutes in a saucepan of water. If you like, add 1/2 cup of isopropyl alcohol to eliminate the cooked cabbage smell. When the liquid is cool, strain it into the large glass jar.

2. Line up the seven small jars (baby jars work well) and pour about one inch of the cabbage juice into each of them. Then fill the drinking glass about halfway with the distilled water.

3. With the eyedropper, add vinegar or lemon juice to one of the jars until the cabbage juice changes color. Use the marker to label the jar "vinegar" or "lemon juice." Both these liquids are strong acids. (Remember to rinse the eyedropper after each use by drawing distilled water from the drinking glass into the dropper and squirting it out into the bowl.)

4. Now add 1/2 teaspoon of baking soda to another jar. Label this jar, too. Baking soda is a strong base and represents the other end of the pH spectrum.

5. Use the eyedropper to add distilled water to another small jar until you see a color change. Label this jar as you did the others. Distilled water is neutral. (The chlorine and fluoride in tap water alters pH.)

6. You now know the color changes that cabbage juice goes through when you add an acid, a base or a neutral substance. Collect and test four other items from around the house.

7. When you've finished testing, line up the seven jars and rank the ingredients from the strongest acid to the strongest base.