Summer Science: Cabbage Paper Indicator

This is the first Summer Science activity here on Slacker Saver. When you give it a try, please let us know in the comments and share your experience. Also, if you have questions or problems, let me know in the comments or here and I’ll do my best to help you.

This is one of my favorite easy chemistry activities. It’s usually considered an elementary activity, but it’s adaptable enough that I used it several years with high school students. It was an engaging introduction to acid and bases for sixteen-year-olds. They were even interested enough to want to take the supplies home for extra exploration (without asking for extra credit)!

The one item that you will likely need to purchase for this activity is a red cabbage. Red cabbages usually look purple (I didn’t name them), and they can be purchased in almost any grocery store’s produce section.

Time required: About thirty minutes (mostly unattended), then time for some paper to dry, and then another twenty minutes. Or another hour, if your kids really like it and you’re having fun–this might actually happen.

Today’s topic is acids and bases. Whether or not the kids know anything about acids and bases, they can have fun with this. It might stink a little bit at the beginning, but I’ve adapted it to get rid of the stink quickly. Actually, it just smells like boiled cabbage. If you like boiled cabbage, you’ll get hungry instead of thinking it stinks. Although this activity involves “chemicals,” there’s nothing dangerous about it. I’ve put a few safety notes at points that may take some safety consideration.

Cabbage Paper Indicator

In chemistry, an “indicator” is a substance that changes in response to another chemical. In this case, our indicator will change color in response to an acid or base.

This activity uses paper strips for convenience, but the straight cabbage juice has pretty much the same color changes. Some of the Topics to Explore would be enhanced with the straight juice.

Method:

1. Get some red cabbage and boil it in water. You don’t need the whole cabbage, several leaves/one cup/one serving will work. Rip/shred/peel the leaves apart. Use just enough water to cover the cabbage. Measurements aren’t important in this activity–summer science is fun like that.[warning]This step involves heat and hot liquids. Use caution with young children.[/warning]

2. When the water has turned dark purple, turn off the heat and let it cool a few minutes. This will take roughly fifteen minutes. The color should be more the strength of coffee than tea. The more water you have compared to cabbage, the longer it will take. If you’re not sure if it’s done, let it boil another five minutes and see if it’s darker. The cabbage should be lighter than the water when you’re done. It doesn’t have to cool completely, just long enough that you’re able to do the next two steps.

3. Take out the cabbage, saving the water. A strainer would work for this. Throw away the cabbage, eat it, or whatever, it’s not important anymore. The purple water is the important part. Actually, sometimes the water looks a little bit more blue than purple, and that’s totally fine.

4. Dip three or four coffee filters (or a couple paper towels) into the purple water, and place them flat on a non-absorbent surface to dry. A cookie sheet is a good choice for the drying part. If the water is still pretty hot, use tongs to dip the paper. Don’t wring out the filters. The drying part could take a while. Have patience. The paper towels will take longer to dry. The leftover cabbage juice is not needed, although you may want to keep it in the refrigerator until you’re done with the activity, for extras and such. You might think up some fun uses.

5. Gather some household substances to test. Twenty substances would be good. Here’s the general rule for the substances: They need to be either a liquid, or something that can be dissolved in a liquid. Also, the less color the substances have, the better. A tablespoon or two of each substance is plenty. Some ideas:

• Any beverage in your refrigerator
• Anything in a bottle in your refrigerator
• Anything in your cupboards that dissolves in water (baking soda, salt, sugar…don’t know if it will dissolve? Try it!)
• Things in bottles in the bathroom (shampoo, body wash, soap…)
• Cleaning supplies[warning]Pay attention to what your kids find here. For example, first graders should not use bleach.[/warning]

6. Dissolve any non-liquid substance to test into a couple tablespoons of water.

7. When the coffee filters (or paper towels) are dry, cut them into strips.These are now your “cabbage paper strips.” The strips should be roughly 1/2″ x 4″, but, again, you don’t need to measure. To estimate an acceptable size, they’ll be about the size of an adult index finger.

8. Start dipping the cabbage paper strips into the household substances. Use each strip for one test, and keep track of which strips go with which substances. You can write on the strips with pencil to help stay organized.

Don’t just dip the strips, though. Look through these questions or mini-activities and do the age-appropriate portions. Roughly, this list goes from easy to hard, although you might want to jump around a bit.

Topics to Explore

•  Describe the color of each cabbage strip with the best possible color name. (Get out the crayons to help match colors if necessary.)
• Group all the substances with similar colors together and compare what color they made the strips. Which substances surprised you?
• Group substances that make the strips similar colors together, maybe five groups. What common characteristics do many of the substances in each group have? Which substances don’t seem to match the rest in that group? What other substances that you haven’t tested might fit into each group? How could you check whether you’re correct?
• What color do you think the strips would turn if you mixed two substances together before testing? Make a prediction, and then try it.[warning]It’s probably not good to mix cleaning supplies together. Vinegar and bleach-based cleaners are dangerous to mix together, so stick with edibles or substances for cleaning people for this one.[/warning]

• Use the scale on the right to estimate the pH of each substance. The left end of the scale is pH 1, the right end is pH 14, and the middle is 7. You’ll have to estimate, since the picture is missing a few colors.
• A substance with a pH of less than 7 is an acid, one with a pH of more than 7 is a base. Group the substances into acids and bases, and make a list of common characteristics in each group. Which characteristics do both groups have? Which characteristics are opposites?
• Think of an illusion (“magic trick”) you could use to surprise your friends or family with the color-changing paper strips or the straight cabbage juice. Try it.
• Look up why the “p” of pH is lower-case, but the “H” isn’t.
• Purchase some commercial pH test strips (online, or check your local pool supply store) and check how close your pH estimates are.

This is just a start for acid and base concepts, there’s a lot more your children could study if they’re really into this. The Internet has a lot of reliable sources for many questions you may have developed during this activity, search away.

What cool observations do you have? Questions? Suggestions for Topics to Explore? Let us know in the comments.

Linked at A Bowl Full of Lemons.