Hydroxide
The hydroxides of the transition metals and post-transition metals usually have the metal in the +2 (M = Mn, Fe, Co, Ni, Cu, Zn) or +3 (M = Fe, Ru, Rh, Ir) oxidation state. None are soluble in water, and many are poorly defined. One complicating feature of the hydroxides is their tendency to undergo further condensation to the oxides, a process called olation. Hydroxides of metals in the +1 oxidation state are also poorly defined or unstable. For example, silver hydroxide Ag(OH) decomposes spontaneously to the oxide (AgO). Copper(I) and gold(I) hydroxides are also unstable, although stable adducts of CuOH and AuOH are known. The polymeric compounds M(OH) and M(OH) are in general prepared by increasing the pH of an aqueous solutions of the corresponding metal cations until the hydroxide precipitates out of solution. On the converse, the hydroxides dissolve in acidic solution. Zinc hydroxide Zn(OH) is amphoteric, forming the tetrahydroxidozincate ion in strongly alkaline solution.
Aluminium hydroxide
Gibbsite is also known as hydrargillite, named after the Greek words for water (') and clay ('). The first compound named "hydrargillite" was thought to be aluminium hydroxide, but was later found to be aluminium phosphate; despite this, both "gibbsite" and "hydrargillite" are used to refer to the same polymorphism of aluminium hydroxide, with "gibbsite" used most commonly in the United States and "hydrargillite" used more often in Europe. In 1930, it was referred to as "α-alumina trihydrate" to contrast it with bayerite, which was called "β-alumina trihydrate" (the alpha and beta designations were used to differentiate the more- and less-common forms respectively). In 1957, a symposium on alumina nomenclature attempted to develop a universal standard, resulting in gibbsite being designated γ-Al(OH), bayerite becoming α-Al(OH), and nordstrandite being designated Al(OH). Based on their crystallographic properties, a suggested nomenclature and designation is for gibbsite to be α-Al(OH), bayerite to be designated β-Al(OH), and both nordstrandite and doyleite are designated Al(OH). Under this designation, the α and β prefixes refer to hexagonal, close-packed structures and altered or dehydrated polymorphisms respectively, with no differentiation between nordstrandite and doyleite.