Modern inorganic chemistry

GROUP VII: THE HALOGENS 349

2. If a chloride is heated with manganese(IV) oxide and concen-
   trated sulphuric acid, chlorine is evolved.


3. If the chloride is heated with sodium or potassium dichromate-
   (VI) and concentrated sulphuric acid, a red gas, chromium(VI)
   dichloride dioxide, CrO 2 Cl 2 , is evolved; if this is passed into water,
   a yellow solution of a chromate( VI) is formed.

TESTS FOR BROMIDE

1. Addition of silver nitrate to a solution of a bromide in nitric acid
   produces a cream-coloured precipitate of silver bromide, soluble in
   ammonia (but not so readily as silver chloride). The reaction may
   be used quantitatively, as for a chloride.


2. Addition of concentrated sulphuric acid to a solid bromide
   produces hydrobromic acid, but also some bromine (brown vapour).


3. Addition of chlorine water to a bromide solution liberates
   bromine, winch colours the solution brown.

TESTS FOR IODIDE

1. Addition of silver nitrate to a solution of an iodide in dilute
   nitric acid, yields a yellow precipitate of silver iodide practically
   insoluble in ammonia.


2. Addition of an oxidising agent to a solution of an iodide (for
   example concentrated sulphuric acid, hydrogen peroxide, potassium
   dichromate) yields iodine; the iodine can be recognised by ex-
   tracting the solution with carbon tetrachloride which gives a purple
   solution of iodine.


3. Addition of mercury(II) chloride solution to a solution of an
   iodide gives a scarlet precipitate of mercury(II) iodide, soluble in
   excess of iodide:

21- 4- HgCl 2 -> HgI 2 i + 2Cr

IiKlication of the presence of a given halide ion can be obtained
by the series of tests given in Table 11.4. Confirmatory tests can
then be performed.