Synthetic Inorganic Chemistry

METATHESIS 113

water gives a red suspension. Upon adding HC1 this red powder

very slowly dissolves (much more slowly than copper oxide), and

finally a clear yellow solution of ferric salt is obtained

Fe 2 O 3 + 6HC1 -> 2FeCl 3 + 3H 2 O

Although we are very uncertain about the intermediate forma-
tion of definite hydroxides we are certain that the water in which
the metal oxides is suspended contains metal ions and hydroxide
ions, and we can represent the equilibrium condition by the rever-
sible reactions:

CuO + H 2 O ^ Cu(OH) 2 ^ Cu++ 2OH"

Fe 2 O 3 + 3H 2 O ^± 2Fe(OH) 3 ^ 2Fe+++ 6OH"

The removal of OH~ by acids allows the reactions to run com-
pletely to the right and the metal oxides to dissolve completely.
Precipitation of Metal Hydroxides. In general the addition
of a soluble base to the solution of a metal salt produces a precip-
itate. This precipitate is of rather variable composition, but its
nature is best understood if it is regarded as the hydroxide of the
metal. Thus, sodium hydroxide added to copper sulphate solu-
tion gives a light blue voluminous precipitate and sodium hydrox-

Cu++ SO 4 "" Fe+++ 3C1"

2OH" 2Na+ 3OH" 3Na+

1 1

Cu(OH) 2 j Fe(OH) 3 |

ide added to ferric chloride gives a voluminous reddish brown
precipitate. These precipitates which we have designated as the
hydroxides, Cu(OH) 2 and Fe(OH) 3 , certainly" consist of the metal
oxides in combination with water, although whether it is the definite
amount to form these definite hydroxides* is rather doubtful.
These fresh precipitates will dissolve instantly in dilute acids
instead of slowly as do the anhydrous oxides. Furthermore these
precipitates if separated on a filter and dried will lose all their
water content and revert to the oxides. The light blue copper
hydroxide even when suspended in water will turn black at the
boiling temperature, indicating at least a partial loss of water.

• These precipitates also contain variable amounts of the metal salts,
  that is, they are basic salts in reality. It is better, however, to ignore this
  fact at present as it tends to confuse the argument.