ANDREW LAMBERT PHOTOGRAPHY / SCIENCE PHOTO LIBRARY ANDREW LAMBERT PHOTOGRAPHY / SCIENCE PHOTO LIBRARY
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. Bases form alkaline solutions. The colour range here shows pH, from left to right: 1 (red), 4 (pink), 7 (purple), 10 (green), 13 (yellow). 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.
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