A colourless highly flammable liquid aldehyde, CH3CHO; r.d. 0.78; m.p. –121°C; b.p. 20.8°C. It is made from ethene by the Wacker process and used as a starting material for making many organic compounds. The compound polymerizes if dilute acid is added to give ethanal trimer (or paraldehyde), which contains a six-membered ring of alternating carbon and oxygen atoms with a hydrogen atom and a methyl group attached to each carbon atom. It is used as a drug for inducing sleep. Addition of dilute acid below 0°C gives ethanal tetramer (or metaldehyde), which has a similar structure to the trimer but with an eight-membered ring. It is used as a solid fuel in portable stoves and in slug pellets.