omium hydroxide.~ When heated gently chromic
hydroxide loses a part of its oxygen and hydrogen, forming the
substance CrO.OH, which, like the corresponding aluminium
compound, has more pronounced acid properties than the
hydroxide. It forms a series of salts very similar to the
spinels; chromite is the ferrous salt of this acid, having the
formula Fe(CrO_{2})_{2}. When heated to a higher temperature
chromic hydroxide is completely dehydrated, forming the
trioxide Cr_{2}O_{3}. This resembles the corresponding oxides
of aluminium and iron in many respects. It is a bright green
powder, and when ignited strongly becomes almost insoluble in
acids, as is also the case with aluminium oxide.

~Chromic sulphate~ (Cr_{2}(SO_{4})_{3}). This compound is a violet-colored
solid which dissolves in water, forming a solution of the same color.
This solution, however, turns green on heating, owing to the formation
of basic salts. Chromic sulphate, like ferric and aluminium sulphates,
unites with the sulphates of the alkali metals to form alums, of which
the best known are potassium chrome alum (KCr(SO_{4})_{2}.12H_{2}O) and
ammonium chrome alum (NH_{4}Cr(SO_{4})_{2}.12H_{2}O).

These form beautiful dark purple crystals and have some practical uses
in the tanning industry and in photography. A number of the salts of
chromium are also used in the dyeing industry, for they hydrolyze like
aluminium salts and the hydroxide forms a good mordant.

~Hydrolysis of chromium salts.~ When ammonium sulphide is added
to a solution of a chromium salt, such as the sulphate,
chromium hydroxide precipitates instead of the sulphide. This
is due to the fact that chromic sulphide, like aluminium
sulphide, hydrolyzes in the presence of water, forming chromic
hydroxide and hydrosulphuric acid. Similarly, a soluble
carbonate precipitates a basic carbonate of chromium.

~Compounds containing chromium as an acid-forming element.~ Like
manganese, chromium forms two unstable acids, namely, chromic acid and
dichromic acid. Their salts, the chromates and dichromates, are
important compounds.

~Chromates.~ When a chromium compound is fused with an alkali and an
oxidizing agent a chromate is produced. When potassium hydroxide is used
as the alkali the equation is

2Cr(OH)_{3} + 4KOH + 3O = 2K_{2}CrO_{4} + 5H_{2}O.

This reaction recalls the formation of a manganate und