Such salts, then, only partly
saturated with acid, are termed basic salts. Thus we have
Al_{2}(OH)_{2}(SO_{4})_{2}, Al_{2}(OH)_{4}SO_{4}, as well as
Al_{2}(SO_{4})_{3}, and we can get these basic salts by treating the
normal sulphate [Al_{2}(SO_{4})_{3}] with sufficient caustic soda to
remove the necessary quantities of sulphuric acid. Now it is a curious
thing that of these aluminium sulphates the fully saturated one,
Al_{2}(SO_{4})_{3}, is the most stable, for even on long boiling of its
solution in water it suffers no change, but the more basic is the
sulphate the less stable it becomes, and so the more easily it
decomposes on heating or boiling its solution, giving a deposit or
precipitate of a still more basic sulphate, or of hydrated alumina
itself, Al_{2}(OH)_{6}, until we arrive at the salt
Al_{2}(SO_{4})_{2}(OH)_{2}, which is quite unstable on boiling;
Al_{2}(SO_{4})(OH)_{4} would be more unstable still. This behaviour may
be easily shown experimentally. We will dissolve some "cake alum" or
normal sulphate of alumina, Al_{2}(SO_{4})_{3}, in water, and boil some
of the solution.
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