Modern inorganic chemistry

420 THE TRANSITION ELEMENTS

character. In the hydrated salt, and in solution, species such as

[Zn(H 2 O) 6 ]^2 + exist; the latter is slightly acidic, forming

[Zn(H 2 O) 5 OH] +. In presence of excess chloride ion, tetrahedral

complexes such as [ZnCl 4 ]^2 ~ may be formed. Other important

zinc salts are the hydrated sulphate ZnSO 4 .7H 2 O, isomorphous

with the corresponding hydrated sulphates of, for example, iron(II)

and nickel, and often used as a source of Zn^2 *(aq), and the sulphide,

ZnS, obtained as a white precipitate when hydrogen sulphide is

passed through a solution of a zinc(II) salt in presence of ammonia

and ammonium chloride.

COMPLEXES

The aquo-complex [Zn(H 2 O) 6 ]2+ and the tetrahedral [ZnCl 4 ]^2 "^
have already been mentioned. Numerous hydroxo-complexes, foi
example [Zn(OH) 6 ]^4 ~, [Zn(OH) 4 ]^2 ~, have been described. Addition
of excess ammonia to an aqueous Zn(II) solution produces the
tetraamminozinc cation [Zn(NH 3 ) 4 ]2+. Hence zinc tends to form
4-coordinate, tetrahedral or (less commonly) 6-coordinate octahedral
complexes.

TESTS FOR ZINC

1. Alkali hydroxide gives a white precipitate soluble in excess. The
   white precipitate, Zn(OH) 2 , gives the oxide when dehydrated; the
   white ^ yellow reversible colour change observed on heating the
   oxide is a useful confirmatory test.


2. Addition of sulphide ion to a solution of a zinc salt containing
   ammonia and ammonium chloride gives a white precipitate of zinc
   sulphide.

QUESTIONS

1. Explain the following observations, giving equations wherever
   possible.
   Anhydrous cupric sulphate is white but forms a blue hydrate and
   a blue aqueous solution. The solution turns yellow when treated
   with concentrated hydrochloric acid, dark blue with ammonia, and
   gives a white precipitate and brown solution when treated with
   potassium iodide. A yellow-brown aqueous solution of ferric
   chloride becomes paler on acidification with sulphuric or nitric