#O=CH_2#. This has appreciable water solubility (approx. #40*g*L^-1#) and such solutions are known as #"formalin"#, which is widely used to preserve tissue in the undertaking caper.
#"Formaldehyde"# has a normal boiling point of #-19# #""^@C#. It is a quite unpleasant gas to get a whiff of.