Iron(II) hydroxide
Iron(II) hydroxide or ferrous hydroxide is an inorganic compound with the formula Fe(OH). It is produced when iron(II) salts, from a compound such as iron(II) sulfate, are treated with hydroxide ions. Iron(II) hydroxide is a white solid, but even traces of oxygen impart a greenish tinge. The air-oxidized solid is sometimes known as "green rust".
Oxide
The oxide ion has the formula O. It is the conjugate base of the hydroxide ion, OH and is encountered in ionic solids such as calcium oxide. O is unstable in aqueous solution − its affinity for H is so great (p"K" ~ −38) that it abstracts a proton from a solvent HO molecule: