A colourless liquid water-soluble acid, C3H7COOH; r.d. 0.96; b.p. 163°C. It is a weak acid (Ka = 1.5 × 10−5 mol dm−3 at 25°C) with a rancid odour. Its esters are present in butter and in human perspiration. The acid is used to make esters for flavourings and perfumery.