Oxygen is a powerful oxidizing agent; after all, one can say that the process of oxidation is named after it! The reason why this is so is because of its high electronegativity. Upon reacting with other species, it will undergo reduction and take on a negative oxidation state, oxidizing its partner in the process. The only exception is in its reaction with fluorine, which is more electronegative than oxygen is (see rules for assigning oxidation numbers above). Because of the relative abundance of oxygen relative to fluorine, however, the “electron-grabbing” effect of oxygen is much more evident in everyday life. Rusting, for example, occurs when iron is oxidized to ferric oxide (Fe2O3) and complexes with water molecules to form a hydrate (Fe2O3•xH2O where x is the number of water molecules to which it is complexed and may vary).
Oxide is the general name usually given to binary compounds in which the oxygen is in the −2 oxidation state (as distinguished from peroxides and superoxides, for example). Certain oxides dissolve in water to give acidic solutions; such oxides are called acidic anhydrides. Other oxides may dissolve in water to give basic solutions; such oxides are called basic anhydrides. Acidic anhydrides are mostly oxides of nonmetals, such as SO3, which dissolves in water to give sulfuric acid, H2SO4. Oxides of Groups IA and IIA metals, on the other hand, tend to be basic anhydrides, such as BaO and CaO.
Instead of thinking of anhydrides in terms of what they would do in water, one can also think in the opposite direction: Anhydrides are obtained by the removal of water from acidic and basic compounds. CaO, for example, can be obtained by removing water from calcium hydroxide, Ca(OH)2:
Ca(OH)2 (s) → CaO (s) + H2O (l)
Acidic and basic anhydrides are Lewis acids and bases.