Some substances are classified as either an acid or a base. Think of acids and bases as opposites - acids have a low pH and bases have a high pH. For reference, water (a neutral) has a pH of 7 on a scale of 0–14. Scientists can tell whether a substance is an acid or a base using an indicator. An indicator is usually a chemical that changes color if it comes into contact with an acid or a base.
As you can see, red cabbage juice turns purple when mixed with something acidic like lemon juice and turns green when mixed with something basic like baking powder. Red cabbage juice is considered to be an indicator because it shows us something about the chemical composition of other substances.
What is it about red cabbage that makes this happen? Red cabbage contains a pigment called anthocyanin that changes color when mixed with an acid or a base. The pigment turns red in acidic environments with a pH less than 7, and the pigment turns blue-green in alkaline (basic) environments with a pH greater than 7.
Red cabbage is just one of many indicators available to researchers. Some indicators start out colorless and turn blue or pink, for example, when mixed with a base. If there is no color change at all, the substance you are testing is probably neutral, just like water.
