Formaldehyde (HCHO), also called methanal, an organic compound, the simplest of the aldehydes, used in large amounts in a variety of chemical manufacturing processes. It is produced principally by the vapour-phase oxidation of methanol and is commonly sold as formalin, a 37 percent aqueous solution. Formalin may be dehydrated to trioxane, a crystalline trimer, or to an amorphouspolymer, paraformaldehyde, which is a convenient source of gaseous formaldehyde.