Acetic acid, also known as ethanoic acid, is an organic chemical compound, giving vinegar its sour taste and pungent smell. Its structural formula is represented as C₂H₄O₂. Pure, water-free acetic acid (glacial acetic acid) is a colourless liquid that absorbs water from the environment (hygroscopy), and freezes below 16.7 °C (62 °F) to a colourless crystalline solid. Acetic acid is corrosive, and its vapour causes irritation to the eyes, a dry and burning nose, sore throat and congestion to the lungs. It is a weak acid because at standard conditions for temperature and pressure the dissociated acid exists in equilibrium with the undissociated form in aqueous solutions, in contrast to strong acids, which are fully dissociated.

Acetic acid is one of the simplest carboxylic acids (the second-simplest, next to formic acid). It is an important chemical reagent and industrial chemical that is used in the production of polyethylene terephthalate mainly used in soft drink bottles; cellulose acetate, mainly for photographic film; and polyvinyl acetate for wood glue, as well as synthetic fibres and fabrics. In households diluted acetic acid is often used in descaling agents. In the food industry acetic acid is used under the food additive code E260 as an acidity regulator.

The global demand of acetic acid is around 6.5 million tonnes per year (Mt/a), of which approximately 1.5 Mt/a is met by recycling; the remainder is manufactured from petrochemical feedstocks or from biological sources.

Nomenclature

The trivial name acetic acid is the most commonly used and officially preferred name by the IUPAC. This name derives from acetum, the Latin word for vinegar, and is related to the word acid itself. The synonym ethanoic acid is a systematic name that is used in introductions to chemical nomenclature.

Glacial acetic acid is a trivial name for water-free acetic acid. Similar to the German name Eisessig (literally, ice-vinegar), the name comes from the ice-like crystals that form slightly below room temperature at 16.7 °C (about 62 °F).

The most common and official abbreviation for acetic acid is AcOH or HOAc where Ac stands for the acetyl group CH₃−C(=O)−;. In the context of acid-base reactions the abbreviation HAc is often used where Ac instead stands for the acetate anion (CH₃COO⁻), although this use is regarded by many as misleading. In either case, the Ac is not to be confused with the abbreviation for the chemical element actinium.

Acetic acid has the empirical formula CH₂O and the molecular formula C₂H₄O₂ or HC₂H₃O₂ (to emphasize the role of the 'active' hydrogen in forming the salt sodium acetate). To better reflect its structure, acetic acid is often written as CH₃-CO₂-H, CH₃COOH, CH₃CO₂H, or HOCOCH₃. The ion resulting from loss of H⁺ from acetic acid is the acetate anion. The name acetate can also refer to a salt containing this anion, or an ester of acetic acid.