Acids and Bases

1. To make the indicator, take one or two leaves from the cabbage and put them in the blender.
2. Add about 1 cup of water for each leaf you use. Make sure to put the lid on the blender!
3. Blend the cabbage into the water for about 30 seconds.
4. Pour the liquid through a strainer into a container that is big enough for all the liquid.
5. Compost or throw away the cabbage bits.

1. Now that you have an indicator liquid, you can use it to test how acidic or basic something is. Collect some things to test. Lemons, limes, oranges, and kiwis are good fruits to test. You should also collect some items like soap, vinegar, baking soda, sugar, and salt.
2. Make a prediction: Which things that you collected are acidic, and which things are basic? Which things might be neutral? Try sorting things into groups based on your prediction.
3. Pour a little of the indicator into a smaller container – something like an ice cube tray will let you compare lots of tests next to each other, or several small cups works well too.
4. If you are testing a powder, like baking soda, sugar, or salt, stir a little bit of it into the indicator in your container. If you are testing a liquid, add a few drops to the indicator and stir it in.
5. Observe what happens to the indicator. Try testing lots of different substances and see what happens.
    

You should see the indicator change color depending on what substance was added to it. If a substance is acidic, it will make the indicator turn pink or red. If a substance is basic, the indicator will turn lighter blue or green. Cabbage juice works well as an indicator because it contains a pigment called anthocyanin, which dissolves in water, and looks different when exposed to acids and bases. If the acid or base is strong, more of it will interact with the anthocyanin and the color will change more dramatically. If the acid or base is weaker (close to neutral), then the pigment will change less.

Don’t forget to clean up when you are done. Any of the suggested testing substances can be washed down the drain or thrown away. Any leftover indicator can be poured down the drain. Wash any containers that you used, as well as the blender and strainer. Put any leftover substances you tested back where you found them.

One fun thing to explore with pH indicators is the fact that liquids can become more acidic or more basic based on gases that get mixed into them. One way to mix a gas into a liquid is to seal it into a bottle and shake it. That’s not the only way though, and in fact blowing bubbles can work really well. If you blow bubbles into water with your cabbage indicator, does it change color? Whether it does or doesn’t, what does that say about the effect of the gasses you exhale? Do they make the water more acidic or basic, or have no effect? If there is a change, think about what you know about the gasses that we exhale, and see if you can come to any conclusions about their effect on a liquid. Of course, you can do some research and see if your conclusions match what other scientists have found.

One other thing to know is that cabbage isn’t the only plant with anthocyanins. In fact, they’re very common in lots of plants, and even a lot of different foods. Fruits and seeds that are purple or purple-ish are a good place to look, such as various berries, grapes, purple corn, or black beans. Some red and blue foods also have them, like raspberries, cherries, and blue corn. Another place to look is leaves that turn red or purple-ish in the fall, since those colors are a sign of anthocyanins. Similarly, flowers with these colors are also a good place to look. Some of these plants might not work as well as cabbage for making a pH indicator, especially if the plant itself makes the liquid acidic or basic, but you can use the same process you used here to try them out.