Acetic acid , systematically named ethanoic acid , is an acidic, colourless liquid and organic compound with the chemical formula (also written as , , or ). Vinegar is at least 4% acetic acid by volume, making acetic acid the main component of vinegar apart from water and other trace elements. Acetic acid is the second simplest carboxylic acid (after formic acid). It is an important chemical reagent and industrial chemical, used primarily in the production of cellulose acetate for photographic film, polyvinyl acetate for wood glue, and synthetic fibres and fabrics. In households, diluted acetic acid is often used in descaling agents. In the food industry, acetic acid is controlled by the food additive code E260 as an acidity regulator and as a condiment. In biochemistry, the acetyl group, derived from acetic acid, is fundamental to all forms of life. When bound to coenzyme A, it is central to the metabolism of carbohydrates and fats. The global demand for acetic acid is about 6.5 million metric tons per year (t/a), of which approximately 1.5 t/a is met by recycling; the remainder is manufactured from methanol. Vinegar is mostly dilute acetic acid, often produced by fermentation and subsequent oxidation of ethanol. Nomenclature 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. The name "acetic acid" derives from the Latin word for vinegar, "", 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 ice-like crystals that form slightly below room temperature at (the presence of 0.1% water lowers its melting point by 0.2 °C). A common symbol for acetic acid is AcOH, where Ac is the pseudoelement symbol representing the acetyl group ; the conjugate base, acetate (), is thus represented as . (The symbol Ac for the acetyl functional group is not to be confused with the symbol Ac for the element actinium; the context prevents confusion among organic chemists). To better reflect its structure, acetic acid is often written as , , , and . In the context of acid–base reactions, the abbreviation HAc is sometimes used, where Ac in this case is a symbol for acetate (rather than acetyl). Acetate is the ion resulting