THL ELEMENTS OF GROUPS IB AND liB 439chloro-complexes.) In both solid HgI 2 and the complex [HgI 4 ]^2 ~
the mercury is tetrahedrally 4-co-ordinated. The [HgI 4 ]^2 " ion has a
characteristic reaction with ammonia—a trace produces a yellow
colour and more ammonia gives a brown precipitate. (An alkaline
solution containing [HgI 4 ]^2 ~ ions is therefore used as a test for
ammonia; it is sometimes called Messier's reagent.) Insoluble salts
of the anion [HgI 4 ]^2 ~ are known, for example Cu 2 [HgI 4 ] (red).
TESTS FOR MERCURY
Mercury(I) compounds in solution give a white precipitate with
chloride ion, blackened by ammonia (p. 437); alkalis and reducing
agents generally produce black or grey mercury from mercury(I)
compounds.
Mercury(II) compounds in solution give a black precipitate with
hydrogen sulphide or a yellow precipitate with alkali hydroxide
(pp. 437. 438).
Any solid mercury compound when fused with sodium carbonate
yields a grey deposit of mercury. (Caution: mercury vapour is
formed.)
QUESTIONS
- How would you prepare a specimen of copper(II) sulphate,
starting from copper? Indicate the methods you might use to
obtain dry crystals of the pentahydrate. What is the structure of the
latter? (Liverpool B.Sc., Part I) - Explain the following observations:
(a) Copper(I) salts disproportionate in solution, but silver(I) salts
do not.
(b) Silver chloride is insoluble in water, but is soluble in dilute
ammonia.
(c) Copper, silver and gold were all used in ancient times, but
aluminium was not used until recent times. - Give the name and formula of one ore of mercury. How is the
metal (a) extracted from this ore, (b) purified? Starting from the
metal how would you prepare specimens of (c) mercury(I) chloride,
(d) mercury(II) chloride? What deductions have been made from a
study of the vapour density of mercury(I) chloride at different
temperatures? (L, A)