Drop the chopped-up cabbage into a blender and add a cup or two of water. Blend until the water is a nice purple color. Once you have your purple juice, pour some into each of your two bowls. If you don't want any cabbage chunks floating around, pour your juice through a strainer. Try other variations of mixing your vinegar and baking-soda or try adding other liquids or chemicals.
Some chemicals are acids.
Some chemicals are bases. And some chemicals are in between acids and bases and are called neutral. Cabbage juice is naturally neutral. When it is neutral, it is a purplish color. If an acid is poured into it, it will turn reddish.