In chemistry, natural or synthetic substance that changes color in response to the nature of its chemical environment. Indicators are used to provide information about the degree of acidity of a substance (pH) or the state of some chemical reaction within a solution being tested or analyzed. One of the oldest indicators is litmus, a vegetable dye that turns red in acid solutions and blue in basic ones. Other indicators include alizarin, methyl red, and phenolphthalein, each one being useful for a particular range of acidity or a certain type of chemical reaction.

Litmus, vegetable dye obtained from lichens, usually of the genus Variolaria, and used in chemistry to determine the presence of acids and bases in a solution. Strips of paper impregnated with a blue or red litmus solution, or small quantities of the solution itself, are used to indicate the presence of an acid or a base; acids turn blue litmus red, and bases turn red litmus blue.

pH, term indicating the hydrogen ion (positively charged hydrogen atom) concentration of a solution, a measure of the solution’s acidity. Hydrogen ions are usually represented by the symbol H+. The term (from French pouvoir hydrogène, “hydrogen power”) is defined as the negative logarithm of the concentration of H+ ions: pH = -log10[H+], where [H+] is the concentration of H+ ions in moles per litre. Because H+ ions associate with water molecules to form hydronium (H3O+) ions, pH also is often expressed in terms of the concentration of hydronium ions.

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