a jar of pickled red cabbage from the grocery
vinegar
baking soda
a drinking glass

Traditionally, to do this experiment, you need a fresh, red cabbage. You grate up the cabbage and then use one of several methods to extract the juice from it. Instead, we will just open the jar of pickled red cabbage and drain the juice from that into a glass. You can save the cabbage to have with your supper. Be sure to at least try a taste, as many people like it. I know I do.

Pour some of the juice into a saucer. Now comes the fun part. Sprinkle just a tiny pinch of baking soda into the juice. Watch what happens. As soon as the white baking soda hits the red cabbage juice, you get green foam. If you put in much baking soda, you will get a lot of green foam, so be ready to clean up your mess. Eventually, all of your red cabbage juice will turn green. It seems that our experiment is over, but it is not. Now it is time to get out the vinegar. Add a little vinegar to the green cabbage juice. What do you get? Pink foam! Again, there is the potential for lots of mess, so either be careful or have lots of paper towels ready.

Once your cabbage juice is red again, guess what comes next. More baking soda and more green foam. You can keep going back and forth, over and over. Why?

The coloring in the cabbage juice is an acid/base indicator. It is a chemical that changes color. When it is in an acid (such as vinegar), it turns red. When it is in a base (such as baking soda), it turns blue-green. There are many other chemicals that do this, and chemists use them to measure how acid or basic a substance is. You can use it to find out which substances in your refrigerator are acids and which are bases. Divide your cabbage juice into two cups. Use a little vinegar and baking soda, so that the juice in one cup is red and the other is green. Put one drop of each onto a plate. Add a drop of lemon juice to each. What happened? The red juice stayed red. The green juice turned red. That tells us that lemon juice is an acid. Try the same thing with some of the other things in the refrigerator. Just be sure to clean up your mess and be sure that no one eats your experiment.

(Well, I found out that pickled red cabbage is not a worldwide food. If you can't find it, you can get a fresh, red cabbage and chop some of it into a blender with a little water. Blend it until the liquid is very red and then strain it. The liquid will change colors with baking soda and vinegar. You can also test other red and purple fruits and vegetables, as many of them will also change colors, although not as dramatically.)