Formaldehyde Gas at room temperature. Formalin is a 40%aqueous solution. HO H0 OH H-C: H formaldehyde, b.p.-21C formalin trioxane,m.p.62CFormaldehyde Gas at room temperature. Formalin is a 40% aqueous solution. O C O C O C H H H H H H heat H C O H H 2 O H C H OH HO trioxane, m.p. 62  C formaldehyde, b.p. -21  C formalin