Iron(II) oxide is an chemical compound composed of iron in the +2 oxidation state bonded to oxygen atoms. It easily converts to a mixture of iron and iron(III) oxide at room temperature. It is stable though when it is very hot. It is a black powder. If it gets hydrated (bonded to extra water molecules), it turns green. The green chemical, named iron(II) hydroxide, gets oxidized by oxygen to a reddish color of iron(III) oxide.