Any substance that changes color in the presence of an acid or base is called an indicator . Litmus is an indicator. If we are using litmus, we are testing whether something is an acid or a base. Acids are sustances that contain lots of hydrogen ions. The stronger the acid, the more free hydrogen ions in solution.
Acids make Litmus turn red.
So if you dipped blue litmus paper into lemon juice, the paper would turn red. If you dipped red litmus into the lemon juice or vinegar, the red paper would stay red. If you taste a substance that is an acid, like lemon juice or vinegar, it tastes sour. Vinegar, orange juice and grapefruit are all common acids. Strong acids include nitric acid, hydrochloric acid, and sulfuric acid.