Cobalt it reacts slowly with mineral acids, and very slowly with moist, but not with dry, air. Common oxidation states of cobalt include +2 and +3, although compounds with oxidation states ranging from −3 to +5 are also known. A common oxidation state for simple compounds is +2 (cobalt(II)). These salts form the pink-colored metal aquo complex [Co(HO)] in water. Addition of chloride gives the intensely blue . In a borax bead flame test, cobalt shows deep blue in both oxidizing and reducing flames. Several oxides of cobalt are known. Green cobalt(II) oxide (CoO) has rocksalt structure. It is readily oxidized
Cobalt(III) hydroxide Cobalt(III) hydroxide Cobalt(III) hydroxide or cobaltic hydroxide is a chemical compound with formula or . It is an ionic compound, with trivalent cobalt cations and hydroxyl anions . The compound is known in two structurally different forms, "brownish-black" and "green". The brownish-black form is a stable solid and can be prepared by reaction of water solutions of cobalt(II) chloride and sodium hydroxide, followed by oxidation with ozone. The green form, formerly thought to be cobalt(II) peroxide, apparently requires carbon dioxide as a catalyst. It can be prepared by adding hydrogen peroxide to a solution of cobalt(II) chloride in 96% ethanol