Plant acid-base indicator. Red cabbage (lower centre) contains a natural acid-base indicator and shows a range of pH by the colour changes seen. pH is the measure of the amount of hydrogen ions in a solution, and is used to measure its alkalinity or acidity. Bases form alkaline solutions. The colour range here shows pH, from left to right: 1, 4, 7, 10, 13. Acids have a pH less than 7. Alkalis have a pH greater than 7. Neutral solutions are pH 7. Each step of the pH scale marks a ten-fold change in acidity/alkalinity. The chemical in red cabbage that changes colour is an anthocyanin. The loss or gain of hydrogen ions changes its colour.