Some common examples of acid-base indicators include blue grapes, which can change color from deep red in an acid to violet in a base; beets, which change from red to purplish when in a very basic substance; and blueberries, which turn red in strong acids. While these are common examples, many other things can be used as an acid-base indicator.
Theoretically, if a substance undergoes a chemical change that is reversible when the pH changes, it can be used as a pH indicator. In actual use, however, the indicator must undergo a significant change in a property of the substance that is easily detectable. In most cases, the color of the object is the property that changes. Other objects, however, may have other properties, such as odor, that can change.
Many acid-base indicators are derived from plants. Flowers, fruits and other plant parts that are red, purple or blue contain anthocyanins, chemical compounds that change color if the pH changes.
Baking soda can also be used in a manner similar to an acid-base indicator. It fizzles when it is added to an acid but does not react to a base. The reaction cannot be easily reversed, however. This means that baking soda is more useful as a test to see if acids are present rather than being a true acid-base indicator.