any bright-coloured synthetic organic compound whose molecule contains two groups of atoms—one acidic, such as a carboxylic group, and one colour-producing, such as an azo or nitro group. Acid dyes are usually applied in the form of their sodium salts, chiefly on wool but also on silk and, to a limited extent, in combination with a mordant, or fixing agent, on cotton and rayon. These dyestuffs produce bright, usually fast shades in a wide range of colours.