Has a pH lower than 7. Contains more hydronium ions than hydroxide ions. The farther from 7, the stronger the acid. Contains more hydroxide ions than hydronium ions. The farther from 7, the stronger the base.
Most indicators are weak organic acids or bases that react with ions in solution.
It is this chemical reaction that produces the color change. Indicators are used to determine the approximate pH of a substance. Bases turn red litmus blue. Phenolphthalein - Colorless in an acid. Very faint pink in a neutral solution.