346 HEAVY METALS OF GROUPS VI, VII, AND VIIIThe ease with which carbon dioxide is expelled from NiCO 3 indi-
cates that nickelous oxide, NiO, is a weakly basic oxide. The fact
that NiCO 3 , the salt of weak carbonic acid, exists at all shows
that NiO has distinct, even if rather weak, basic properties. Since
the carbonates CrCO 3 , MnCO 3 , FeCO 3 , CoCO 3 , CuCO 3 , ZnCO 3 ,
PbCO 3 all decompose at about the same temperature the oxides
CrO, MnO, FeO, CoO, CuO, ZnO, PbO are indicated as having a
basic strength of about the same order as that of NiO. The black
oxide which remained after heating NiCO 3 dissolved in HCl to give
a solution of NiCl 2 , but the evolution of free chlorine indicated an
oxidizing agent. Nickelous oxide, NiO, takes on oxygen from the
air to form nickelic oxide, Ni2O 3 , and this is the oxidizing agent
Ni 2 O 3 + 6HC1 -• 2NiCl 2 + 3H 2 O + Cl 2- Non-Existence of Carbonates of Tnvalent Metals.
Dissolve 2 grams of ferric alum in 10 cc. of water and add 2 N
Na 2 CO 3 in excess. Effervescence and a reddish brown,
voluminous precipitate are noted. Collect the precipitate on
a filter and wash it free of Na 2 CO 3 solution. Then treat the
precipitate with 6 N HCl in which it dissolves without effer-
vescence.
The ions of ferric carbonate are brought together in the solu-
tion, but this salt evidently cannot exist. Its hydrolysis products,
ferric hydroxide and carbonic acid (CO 2 ), are obtained. The
behavior of the red precipitate when treated with HCl shows that
it is ferric hydroxide and not ferric carbonate.
Chromic salts and sodium carbonate show the same behavior.
Since the carbonates of trivalent iron and chromium do not exist
the oxides Fe 2 O 3 and Cr 2 O 3 are more weakly basic than FeO and
CrO.
- Oxidation of An Oxide of a Divalent Metal. Heat
0.5 gram of cobalt carbonate in a dry porcelain dish, holding
the dish in crucible tongs and rotating it gently over a small
flame. Do not allow it even to approach a visible red
heat. The light pink cobalt carbonate turns jet black. After
it cools treat this black powder with QN HCl. It dissolves,
and a pronounced odor of chlorine is observed. The solution
is deep blue when concentrated but light pink when diluted.