Formaldehyde is a carcinogenic and highly toxic naturally occurring organic compound with the formula CH2O (H−CHO). The pure compound is a pungent-smelling colorless gas that polymerizes spontaneously into paraformaldehyde, hence it is stored as an aqueous solution (formalin), which is also used to store animal specimens. It is the simplest of the aldehydes (R−CHO). The common name of this substance comes from its similarity and relation to formic acid.