Modern inorganic chemistry

384 THE TRANSITION ELEMENTS

TESTS FOR CHROMATES AND DICHROMATES(VI)

1. Addition of lead(II) nitrate in ethanoic acid solution gives a
   yellow precipitate of lead chromate. PbCrO 4.


2. A reducing agent (for example sulphur dioxide) reduces the
   yellow chromate or orange dichromate to the green chromium(III)
   state.


3. Hydrogen peroxide with a chromate or a dichromate gives a
   blue colour.

TESTS FOR CHROMIUM(III) SALTS (i.e. FOR HYDRATED

Cr^3 + ION) :

1. Addition of alkali gives a green gelatinous precipitate of
   chromium(III) hydroxide, soluble in a large excess of strong alkali.


2. Addition" of sodium peroxide to a solution gives a yellow colour
   of the chromate.

MANGANESE

THE ELEMENT

Manganese is the third most abundant transition metal, and is

widely distributed in the earth's crust. The most important ore is

pyrolusite, manganese(IV) oxide. Reduction of this ore by heating

with aluminium gives an explosive reaction, and the oxide Mn 3 O 4

must be used to obtain the metal. The latter is purified by distillation

in vacuo just above its melting point (1517 K); the pure metal can

also be obtained by electrolysis of aqueous manganese(II) sulphate.

The metal looks like iron; it exists in four allotropic modifications,

stable over various temperature ranges. Although not easily

attacked by air, it is slowly attacked by water and dissolves readily

in dilute acids to give manganese(II) salts. The stable form of the

metal at ordinary temperatures is hard and brittle—hence man

ganese is only of value in alloys, for example in steels (ferroalloys)

and with aluminium, copper and nickel.

COMPOUNDS OF MANGANESE

Although it exhibits a wide range of oxidation states, from -f 1
(corresponding to formal loss of all the outer electrons, 3d^5 4s^2 ) to 0.