mates and dichromates.~ When a dilute solution of
a chromate or dichromate is acidified with an acid, such as sulphuric
acid, no reaction apparently takes place. However, if there is present a
third substance capable of oxidation, the chromium compound gives up a
portion of its oxygen to this substance. Since the chromate changes into
a dichromate in the presence of an acid, it will be sufficient to study
the action of the dichromates alone. The reaction takes place in two
steps. Thus, when a solution of ferrous sulphate is added to a solution
of potassium dichromate acidified with sulphuric acid, the reaction is
expressed by the following equations:

(1) K_{2}Cr_{2}O_{7} + 4H_{2}SO_{4}
= K_{2}SO_{4} + Cr_{2}(SO_{4})_{3} + 4H_{2}O + 3O,

(2) 6FeSO_{4} + 3H_{2}SO_{4} + 3O = 3Fe_{2}(SO_{4})_{3} + 3H_{2}O.

The dichromate decomposes in very much the same way as a permanganate
does, the potassium and chromium being both changed into salts in which
they play the part of metals, while part of the oxygen of the dichromate
is liberated.

By combining equations (1) and (2), the following is obtained:

K_{2}Cr_{2}O_{7} + 7H_{2}SO_{4} + 6FeSO_{4}
= K_{2}SO_{4} + Cr_{2}(SO_{4})_{3} + 3Fe_{2}(SO_{4})_{3} + 7H_{2}0.

This reaction is often employed in the estimation of iron in iron ores.

~Potassium chrome alum.~ It will be noticed that the oxidizing
action of potassium dichromate leaves potassium sulphate and
chromium sulphate as the products of the reaction. On
evaporating the solution these substances crystallize out as
potassium chrome alum, which substance is produced as a
by-product in the industries using potassium dichromate for
oxidizing purposes.

~Chromic anhydride~ (CrO_{3}). When concentrated sulphuric acid is added
to a strong solution of potassium dichromate, and the liquid allowed to
stand, deep red needle-shaped crystals appear which have the formula
CrO_{3}.This oxide of chromium is called chromic anhydride, since it
combines readily with water to form chromic acid:

CrO_{3} + H_{2}O = H_{2}CrO_{4}.

It is therefore analogous to sulphur trioxide which forms sulphuric acid
in a similar way:

SO_{3} + H_{2}O = H_{2}SO_{4}.

Chromic anhydride is a very strong oxidizing agent, giving up oxygen and
forming chromic oxide:

2CrO_{3} = Cr_{2}O_{3} + 3O.

~Rare elements of the family.~ Molybdenum, tungsten, and uranium
ar