When an oxide reacts with water (not all oxides do!), then the reaction product usually is a compound of the form MOx(OH)y. A fully reacted compound
results in M(OH)z. Here x, y, z are small integers, and M is either a metallic, non-metallic or semi-metallic element.
Some examples:
M = Na: Na2O + H2O ---> 2NaOH (lye, strongly basic)
M = S: SO3 + H2O ---> SO2(OH)2 (sulphuric acid, strongly acidic)
M = Ca: CaO + H2O ---> Ca(OH)2 (hydrated calcium lye, strongly basic)
M = N: N2O5 + H2O ---> 2NO2(OH) (nitric acid, strongly acidic)
M = Cr : Cr2O3 + H2O ---> 2Cr(OH)3 (feebly acidic, feebly basic)
The properties of these compounds MOx(OH)y strongly depend on the element M, and on the oxidation state of M.
For non-metallic M, in general, one can say that the compound MOx(OH)y is acidic. It can loose one or more H(+) ions. For metallic M, with low
oxidation state for M (+1, +2), one can say that the compound MOx(OH)y is basic. It accepts H(+) ions in acidic environment, releasing the ion M(+) or
M(2+) as aqueous metal ion.
There is, however, a large group of intermediate elements M, where the compound MOx(OH)y can be both weakly acidic or weakly basic. These intermediate
elements are the semiconductors, but also many metals in higher oxidation states. In strongly acidic environments, such compounds can accept H(+)
ions, resulting in combination of OH(-) ions and H(+) ions. In strongly alkaline environments, these compounds can give off H(+) ions, resulting in
formation of negatively charged ions. Some examples are: chromium in +3 oxidation state, aluminium (which always is in +3 oxidation state), tin in +4
oxidation state, vanadium in +4 oxidation state. Such compounds are called "amphoteric".
Remarkably, many transition metals, which can exist in very high oxidation states (e.g. Cr(VI), Mn(VII), V(V)), react like acids. So, although we have
a metal central atom, the oxides of these at high oxidation states are purely acidic, e.g. M2O7 + H2O --> 2MnO3(OH), which is strongly acidic and
has no basic properties at all.
Finally, I want to say, that there also is a large group of oxides, which is inert. These do not react with water at all under normal conditions. Most
notably are the medium oxidation state transition metal oxides (e.g. Cr2O3, Co3O4, TiO2), but also many other metal oxides (e.g. Al2O3, SnO2). These
oxides do not react with water, nor with strong acids, nor with strong bases. In fact, there is no fun at all with these oxides, because they are so
extremely unreactive. Quite extreme conditions are needed to have these reacting, such as mixing with molten NaOH, or boiling for hours in a
concentrated H2SO4/HNO3/HF mix.
Many of the oxides, I mentioned here as inert, only become so after calcining. This inertness also has many applications, e.g. in ceramics, but also
as pigments of great fastness.
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