Formaldehyde is a naturally occurring organic compound with the formula CH2O (H−CHO). The pure compound is a pungent-smelling colorless gas that polymerises spontaneously into paraformaldehyde (refer to section Forms below), hence it is stored as an aqueous solution (formalin). It is the simplest of the aldehydes (R−CHO). The common name of this substance comes from its similarity and relation to formic acid.