Compounds with the carboxy group, -\mathrm{COOH}, are called carboxylic acids. Ethanoic acid (acetic acid), \mathrm{CH_3COOH}, is the acid in vinegar.
Pure ethanoic acid is a colourless liquid with a sharp smell, and it freezes at about 17 °C. It turns into an ice-like solid in a cold room, so it is called glacial acetic acid.
Vinegar is made when bacteria oxidize the ethanol in wine and other alcoholic liquids. In industry, most ethanoic acid is made from methanol and carbon monoxide.
In water, carboxylic acids ionize and release hydrogen ions, but only a small fraction of their molecules ionize, which makes them weak acids.
In a 0.1 mol/L solution, only about 1 % of the ethanoic acid molecules are ionized, while hydrochloric acid of the same concentration is almost completely ionized. So the pH of the ethanoic acid solution is about 3, compared with 1 for hydrochloric acid.
When ethanoic acid is added to sodium hydrogencarbonate solution, bubbles of carbon dioxide are given off.
\mathrm{CH_3COOH} + \mathrm{NaHCO_3} \longrightarrow \mathrm{CH_3COONa} + \mathrm{H_2O} + \mathrm{CO_2}
This happens because ethanoic acid is a stronger acid than carbonic acid. Phenol, which is a weaker acid than carbonic acid, does not give off carbon dioxide, so this reaction is used to tell carboxylic acids from phenols.
Methanoic acid (formic acid), \mathrm{HCOOH}, is the simplest carboxylic acid. It is found in ants and stinging nettles, and its name comes from the Latin word for ant.
Its molecule contains the same arrangement of atoms as an aldehyde group, so, unlike other carboxylic acids, it is a reducing agent: it gives a silver mirror and turns potassium permanganate solution colourless. It is then oxidized to carbon dioxide and water.
In ethanoic acid vapour and in solvents such as benzene, two molecules of ethanoic acid pair up, joined by two hydrogen bonds between their carboxy groups.
Because of these hydrogen bonds, carboxylic acids have high boiling points. Ethanoic acid boils at 118 °C, higher than propan-1-ol, which has the same relative molecular mass, at 97 °C.
When a molecule of water is removed from two molecules of ethanoic acid using a dehydrating agent, ethanoic anhydride (acetic anhydride) forms. It is used to make medicines such as aspirin.