Conure - 24-1-2024 at 10:25
Ordinary brown rust can be removed from a steel plate in no time with citric acid, but pure red Fe2O3 is much tougher. Even hot 75% H3PO4 takes
minutes to disolve it. At room temperature it takes at least 8h. Why is that? Same with black Fe3O4 btw.
DraconicAcid - 24-1-2024 at 11:08
Because the pure material is calci8ned and generally unreactive.
Admagistr - 24-1-2024 at 15:47
Fe2O3 prepared under heat has the structure of corundum - the alpha modification of Al2O3, which is insoluble in acids. In general, crystalline
substances dissolve much less easily than amorphous or only very finely crystalline substances. In the case of gels precipitated from solutions, how
old the gel is also affects solubility; the older it is, the less soluble it is, as the size of its crystals increases at the microscopic level. Thus,
in practice, some water-insoluble hydroxide is easily dissolved in acid shortly after its preparation, but cannot be dissolved in the same acid six
months later.