THE ELEMENTS OF GROUPS IB AND (IB 435
of a cadmium(II) salt; presence of chloride ion may reduce the
concentration of Cd2+(aq) sufficiently to prevent precipitation.
Complexes of cadmium include, besides those already mentioned,
a tetracyanocadmiate [Cd(CN) 4 ]^2 ~ which in neutral solution is
sufficiently unstable to allow precipitation of cadmium(II) sulphide
by hydrogen sulphide. Octahedral [CdCl 6 ]^4 " ions are known in the
solid state, as, for example, K 4 CdCl 6.TESTS FOR CADMIUM
The reaction of Cd^2 +(aq) with sulphide ion, to give yellow CdS, and
with hydroxide ion to give the white Cd(OH) 2 , soluble in ammonia,
provide two useful tests.MERCURY
THE ELEMENT
Mercury has been known for many centuries, perhaps because its
extraction is easy; it has an almost unique appearance, it readily
displaces gold from its ores and it forms amalgams with many other
metals—all properties which caused the alchemists to regard it as
one of the "fundamental' substances.
It occurs chiefly as cinnabar, the red sulphide HgS, from which it
is readily extracted either by roasting (to give the metal and sulphur
dioxide) or by heating with calcium oxide; the metal distils off and
can be purified by vacuum distillation.
Mercury has a large relative atomic mass, but, like zinc and
cadmium, the bonds in the metal are not strong. These two factors
together may account for the very low melting point and boiling
point of mercury. The low boiling point means that mercury has an
appreciable vapour pressure at room temperature; 1 m^3 of air in
equilibrium with the metal contains 14 mg of vapour, and the latter
is highly toxic. Exposure of mercury metal to any reagent which
produces volatile mercury compounds enhances the toxicity.
The metal is slowly oxidised by air at its boiling point, to give red
mercury(II) oxide; it is attacked by the halogens (which cannot
therefore be collected over mercury) and by nitric acid. (The
reactivity of mercury towards acids is further considered on pp. 436,
438.) It forms amalgams—liquid or solid—with many other metals;
these find uses as reducing agents (for example with sodium, zinc)
and as dental fillings (for example with silver, tin or copper).