The pH of a solution can be approximated with a pH indicator, which is any chemical whose color changes with pH. Many pH indicators are found in plants; the pigment of red cabbage is a good example. This pigment is red at low pH values (acidic), light purple at slightly acidic pH values, blue at neutral pH values, light green at moderately alkaline pH values, and dark green at very alkaline pH values.

Boil shredded red cabbage in water for about 5 minutes. Strain the broth from the cabbage and allow it to cool. Add the cooled blue broth to at least three cups of clear water so that each cup is less than half-filled or to three white porcelain bowls. Add a teaspoon of white vinegar to one cup and a teaspoon of baking soda to the second cup. Watch for color changes. Add nothing to the third cup so that it remains blue.
What color is the red cabbage before being boiled? Are the juices in red cabbage more or less acidic than vinegar? What would happen to the baking soda solution if you were to slowly add vinegar to it? What color would you get if a teaspoon of concentrated broth were added to a glass of water?