Again, the metals precipitated in the acid solution form sulphides which
may be divided into groups, the one consisting of those which are
soluble, and the other of those which are not soluble, in alkalies. This
classification is shown in the following summary:--
1. _Precipitable in an acid solution._
(a) Soluble in Alkalies.--Sulphides of As, Sb, Sn, Au, Pt, Ir, Mo, Te,
and Se.
(b) Insoluble in Alkalies.--Sulphides of Ag, Pb, Hg, Bi, Cu, Cd, Pd, Rh,
Os, and Ru.
2. _Not precipitated in an acid solution, but thrown down in an alkaline
one._
Sulphides of Mn, Zn, Fe, Ni, Co, In, Tl, and Ga.
These can again be divided into those which are dissolved by dilute
acids and those which are not.
3. _Not precipitated in an acid or alkaline solution, but thrown
down on neutralising the latter._
Sulphides of V and W.
Sulphuretted hydrogen is a strong reducing agent. Ferric salts are
thereby quickly reduced to ferrous; in hot solutions nitric acid is
decomposed. These changes are marked by a precipitation of sulphur, and
the student must be careful to pass the gas sufficiently long, and not
be too hasty in concluding that no sulphide will form because it does
not at once make its appearance. The best indication that it has been
passed long enough is the smell of the gas in the solution after
shaking.
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