(or metaformaldehyde), [—CH2O—]3, a trimer of formaldehyde that forms colorless crystals having a characteristic odor. Melting point, 61°–62°C; boiling point, 114°–115°C; density 1.17 g/cm3 at 63°C. It is readily soluble in many organic solvents. With water it forms an azeotropic mixture that contains 70 percent sym-trioxane and boils at 91°C.
Sym-trioxane is obtained by heating a 50–55 percent aqueous solution of formaldehyde with 2 percent sulfuric acid. It decomposes completely into formaldehyde at 150°–180°C. This property permits its use for the production of pure formaldehyde.