1. Chop red cabbage up finely. Boil a pint of water in a saucepan.
2. Add the red cabbage carefully to the boiling water and take the saucepan off the heat. Let it stand for 30 minutes until it is completely cool.
3. Strain the liquid in a jar, and throw away the used cabbage. The liquid should be a dark reddish purple color. Add alcohol to reduce the spoilage of the indicator. (Use a 1 to 5 ratio of alcohol to volume of water.)
4. The color will change when you add acids or bases (alkalis). To test a substance, pour a little of your substance into a small jar. Then add a drop or two of the cabbage juice indicator. A change in color indicates its acidity or basicity.