In chemistry, an indicator is a substance that gives a visible sign, usually by a color change, of the presence or absence of a threshold concentration of a chemical species, such as an acid or an alkali in a solution. Indicators are used to visibly demonstrate chemical changes in a solution. They are usually weak acids or bases, but their conjugate base or acid forms have different colors due to differences in their absorption spectra/Acids_and_Bases/Acid/Indicators). Indicators dissociate slightly in water to form ions, and some examples of indicators are litmus, turmeric, phenolphthalein, and methyl orange. Indicators are classified as acid-base, oxidation-reduction, or specific-substance indicators, and every indicator in each class has a characteristic transition range. The color of an indicator alters when the acidity or the oxidizing strength of the solution, or the concentration of a certain chemical species, reaches a critical range of values.