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Acetic acid, also known as acetic acid (36% - 38%), glacial acetic acid (98%), chemical formula CH3COOH, is an organic monobasic acid, which is the main component of vinegar. Pure anhydrous acetic acid (glacial acetic acid) is a colorless hygroscopic solid with a freezing point of 16.6 ° C (62 ° F). It is colorless crystal after solidification. It is weakly acidic and highly corrosive in aqueous solution. Steam has eyes and nose. Irritating effect.

Acetic acid is widely distributed in nature, for example in fruit or vegetable oils, but mainly in the form of esters. It is present as free acid in the tissues of animals, in excreta and in blood. Many microorganisms can convert different organic substances into acetic acid by fermentation.

Acetic acid is the main component of vinegar, and vinegar almost runs through the entire history of human civilization. Acetic acid-fermenting bacteria (Acetobacter) can be found in every corner of the world. When wine is brewed, it is inevitable that vinegar is found in every country. It is a natural product of these alcoholic beverages after exposure to the air. For example, in China, Dukang’s son, Heita, was given vinegar for his long brewing time.

The crystal structure of acetic acid shows that the intermolecular bonds are dimerized (also called dinuclear junctions) by hydrogen bonding, and the dimer is also present in a vapor state at 120 °C. The dimer has a high stability, and the molecular weight method and the X-ray diffraction have been confirmed by the freezing point reduction, and the carboxylic acid having a smaller molecular weight such as formic acid or acetic acid is present in a solid state and a liquid state, and even in a gaseous form as a dimer. When acetic acid is dissolved in water, the hydrogen bond between the dimers breaks quickly. Other carboxylic acids also have similar dimerization phenomena.