 Ytterbium metal tarnishes slowly in air . Finely dispersed ytterbium readily oxidizes in air and under oxygen . Mixtures of powdered ytterbium with polytetrafluoroethylene or hexachloroethane burn with a luminous emerald @-@ green flame . Ytterbium reacts with hydrogen to form various non @-@ stoichiometric hydrides . Ytterbium dissolves slowly in water , but quickly in acids , liberating hydrogen gas . 
