Acetic acid, systematically named ethanoic acid, is an acidic, colourless liquid and organic compound with the chemical formula CH₃COOH (also written as CH₃CO₂H, C₂H₄O₂, or HC₂H₃O₂). Vinegar is 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.The trivial name “acetic acid” is the most commonly used and preferred IUPAC name. The systematic name “ethanoic acid”, a valid IUPAC name, is constructed according to the substitutive nomenclature.[8] The name “acetic acid” derives from the Latin word for vinegar, “acetum”, which is related to the word “acid” itself.“Glacial acetic acid” is a name for water-free (anhydrous) acetic acid. Similar to the German name “Eisessig” (“ice vinegar”), the name comes from the solid ice-like crystals that form with agitation, slightly below room temperature at 16.6 °C (61.9 °F). Acetic acid can never be truly water-free in an atmosphere that contains water, so the presence of 0.1% water in glacial acetic acid lowers its melting point by 0.2 °C.