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An acid (from the Latin acidus/ac?re meaning sour) is a substance which reacts with a base. Commonly, acids can be identified as tasting sour, reacting with metals such as calcium, and reacting with bases such as sodium carbonate. Aqueous acids have a pH under 7. Solutions with higher acidity have lower pH. Chemicals or substances having the property of an acid are said to be acidic.
Common examples of acids include acetic acid (vinegar is a dilute solution of this liquid), sulfuric acid (used in car batteries), hydrochloric acid (a solution of this gas is used in the stomach for enzymes) and tartaric acid (a solid used in baking). As these examples show, acids can be solutions or pure substances, and can be derived from solids, liquids, or gases. Strong acids and some concentrated weak acids are corrosive, but there are exceptions such as carboranes and boric acid.
There are three common definitions for acids: the Arrhenius definition, the Brønsted-Lowry definition, and the Lewis definition. The Arrhenius definition defines acids as substances which increase the concentration of hydronium ions (H3O+) in solution. The Brønsted-Lowry definition is an expansion: an acid is a substance which can act as a proton donor, while a base acts as a proton acceptor. By this definition, any compound which can easily be deprotonated can be considered an acid. Examples include alcohols and amines which contain O-H or N-H fragments. Lewis acids are electron-pair acceptors, while Lewis bases donate an electron-pair. Examples of Lewis acids include all metal cations, and electron-deficient molecules such as boron trifluoride and aluminium trichloride.

http://en.wikipedia.org/wiki/Acid