The carboxyl hydrogen atoms of acetic acid can be partially ionized into hydrogen ions (protons) and released, resulting in the acidity of the carboxylic acid. Acetic acid is a weak acid in the aqueous solution, with an acidity coefficient of 4.8, pKa=4.75 (25°C), and a 1mol/L acetic acid solution (similar to the concentration of household vinegar) with a pH of 2.4, which means that only 0.4% The acetic acid molecule is dissociated.
The crystal structure of glacial acetic acid shows that the molecules are combined into dimers (also called di-associates) through hydrogen bonding, and the dimers also exist in the steam state at 120°C. The dimer has a higher stability. The molecular weight determination method and X-ray diffraction have been proved by freezing point reduction. The carboxylic acids with lower molecular weights such as formic acid and acetic acid exist in the solid and liquid state, and even in the gaseous state in the form of dimer. When acetic acid and water are dissolved, the hydrogen bond between the dimers will be broken quickly. Other carboxylic acids have similar dimerization phenomena.