Formaldehyde is a naturally occurring organic compound with the formula CH2O and structure H−CHO. The pure compound is a pungent, colorless gas that polymerizes spontaneously into para-formaldehyde, 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.