Acids, like citric acids in citrus fruits, tend to be sour in taste while bases, like baking soda, tend have a bitter taste. But of course you can't taste everything to test if it's an acid! In school you may have used litmus paper to determine if a substance was an acid or a base. In this simple science experiment, you can make your own pH indicator, which tests to see if solutions are acidic or basic.
Grate about 1/2 cup of red cabbage. It might help to first cut the red cabbage into quarters so it's easier to grate.
Put the red cabbage in a bowl and add just enough water to cover it.
Crush the cabbage with the wooden spoon to produce a purple cabbage juice.
Sieve the cabbage juice into several small cups. This is your pH indicator solution.
To determine the pH of the soap, vinegar, or an unknown substance, just add it to the cups. An acid will turn the solution pink, and a base will turn the solution blue.
TL;DR (Too Long; Didn't Read)
Acids and bases cancel each other out, so try reversing the color change by adding the opposite solution.