Formaldehyde Gas at room temperature. Formalin is a 40%aqueous solution. H H h、H-8HE HO _H 0→ OH H-C >H formaldehyde, b.p.-21C formalin trioxane,m.p.62C Formaldehyde ◼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 H2O H C H OH HO trioxane, m.p. 62C formaldehyde, b.p. -21C formalin