Experiment: Red Cabbage pH Indicator
The instructions are laid out here for an adult – please supervise kids as necessary.
1. Get some red cabbage and chop it up into small pieces.
2. Put the chopped cabbage in a pot and cover it with water. Boil for 10 -15 minutes (or until the water becomes well coloured with cabbage juice).
3. Strain off the cabbage juice into a flat dish and let cool for at least an hour.
4. Get some blotting paper or thick watercolour paper (ideally acid-free) and soak in the cabbage juice. Once fully soaked, take it out and leave to dry completely.
5. Now its time to EXPERIMENT! Take some smaller pieces of the cabbage juice-stained paper and brush on different household solutions. Some interesting ones to try are vinegar, cola, baking soda (that’s been dissolved, of course), milk, washing up liquid, laundry liquid, toothpaste, lemon juice, other fruit juices, and anything else that you think might be interesting – just remember to rinse your brush between using different solutions. Older kids might want to try household ammonia, dissolved aspirin and antacids (under adult supervision). Use a pencil to write down under your experiments what solution caused what colour change.
6. With the colour-change knowledge from your experimental stage, paint a red cabbage pH-indicator picture!
And now for the science bit:
Red cabbage contains a water-soluble pigment called flavin which is an anthocyanin. Anthocynins can also be found in apple skins, grapes, poppies and plums; and it changes colour with the pH (or acidity/basicity) of the solution it is in. Very acidic solutions turn it a deep red; neutral solutions make it a purple-ish shade; and basic solutions turn it a greenish-yellow. In this experiment, you are using red cabbage juice as a pH-indicator to find out whether the household solutions you are testing are acidic, basic, or neutral.