Ethanal. Computer illustration of a molecule of ethanal, or acetaldehyde (formula CH3CHO). Atoms are depicted as cylinders and are colour-coded: carbon (yellow), hydrogen (white) and oxygen (red). Ethanal is a colourless liquid with a pungent odour. It is mainly used in the chemical industry, in the preparation of ethanoic acid, ethanoic anhydride and n-butanol.