Acetic acidsiËtɪk/, systematically namedethanoic acidËÉïÉËnoÊɪk/, is an acidic, colourless liquid andorganic compoundwith thechemical formulaCH3COOH(also written asCH3CO2H,C2H4O2, orHC2H3O2).Vinegaris at least 4% acetic acid by volume, making acetic acid the main component of vinegar apart from water. It has been used, as a component of vinegar, throughout history from at least the third century BC.