Acids and bases/Litmus paper
Many vegetable inks contain compounds which can be used for indicator paper.
Simple recipe for litmus paper:
- Chop up some red cabbage
- Add water to cover
- Bring to a boil
- Soak some blotting paper in the colored solution
- Let the paper dry
Experiment with dipping the paper into a variety of common acids and bases. The paper will change color. Other intensely colored vegetables can also be used to make litmus paper.