We'll start with something you have probably seen before: making a pH indicator.
Red cabbage (1 head)
White vinegar (1/2 cup)
Baking soda (1 tsp)
Clear glass containers (at least 3)
Shred the cabbage. Put it in a medium pot and add water until the cabbage is covered. Bring to a boil and then let it cook for about 15 minutes. Really, I just watch it on the stove until the water turns a dark purple. Strain the mixture so you only have the liquid. Do whatever you'd like with the cabbage.
I got almost a quart of indicator from one small head of cabbage. That will keep in the fridge for awhile (my jar has been sitting there for several months.)
While the liquid is cooling, get out three glass containers (or more if you are testing more than baking soda and vinegar.) In each put:
1/2 c water + 1 tsp baking soda (stir to combine)
1/2 c water
1/2 c vinegar
In my pictures, I actually used a cup of water but the color will be more intense if you use less.
Once the indicator has cooled a bit, add 15 drops of it to each of the three containers.
The acid turns pink, the base turns blue, and the water will stay purple (red + blue = purple, acid + base = neutral liquid)
Why? Because cabbage and many other plants contain anthocyanins that change color depending on pH. These substances are what show through in colorful autumn leaves when the chlorophyll drains away. They are also why some plants will have different colors when planted in different soils.
Links for more information:
What are acids, basis, and pH? See if your acids and bases have these properties. Instead of vinegar, I actually added ascorbic acid to water. Ascorbic acid can be purchased through The Baker's Catalogue as a dough enhancer and to make sourdough bread more sour! What about slippery soap--any guesses as to its pH?
How Stuff Works: Where does the color come from in purple cabbage?
Acids and Bases: Frequently Asked Questions (advanced)
Water to Wine: the molecular basis of indicator color changes (advanced. If the chief stewart tasted these liquids, though, he would not comment to the groom that he had saved the best wine for last!)