Why does red cabbage juice change colour with pH?
Molecules called anthocyanins are found in flower petals, leaves, blueberries, blackberries, strawberries, cranberries, cherries, apples and grapes, and in red cabbage. Anthocyanins cause the beautiful red/purple color that you see in roses and in the fall, in maple leaves. Anthocyanins are produced in leaves once chlorophyll (which is green) degenerates in the fall. Nature produces over 300 structurally distinct anthocyanins. By boiling red cabbage leaves, we can take out (extract) the anthocyanins. Anthocyanins dissolved in water (in aqueous solution) change color depending upon the amount of H+ and HO- in the water. (That is the same thing as saying that the absorbance of light by anthocyanins depends on pH.) Flowers such as hydrangea contain anthocyanins, and their color is sensitive to the acidity of the soil. The anthocyanins in red cabbage turn bright pink in acid solutions, reddish-purple in neutral solutions and green in basic solutions. But anthocyanins from other plants have other pH/color profiles.