236 HEAVY METALS OF GROUPS I AND IIExperiments
Review in Chapter III Experiments 14, 15, 18, 19, pages 91-92
and the discussion of the basic properties of metal hydroxides,
page 112, and of complex ions, page 118. In Chapter IV, review
Experiments 16 on page 170 and 22 on page 175.
- Stability of the Carbonates, (a) To about 10 cc. each of
solutions of CuSO 4 , ZnSO 4 , CdCl 2 , and AgNO 3 add 1N Na 2 CO 3
until no further precipitate is formed and notice very carefully
whether any bubbles of gas escape. No gas escapes from the
cadmium and silver salt solutions; effervescence is noted with
the copper and zinc salt solutions. Collect the precipitates
from the copper and zinc salt solutions on niters, and wash
thoroughly with water until the excess of Na 2 CO 3 is removed.
This is accomplished when the washings from the filter no
longer effervesce when HC1 is added. Now pour a few drops
of HC1 on these precipitates and note that they effervesce
when they dissolve.
The escape of carbon dioxide when the Na 2 CO 3 is added shows
that the copper and zinc carbonates hydrolyze, but the further
escape of carbon dioxide when the precipitates are treated with
acid shows that the hydrolysis has not been complete. The pre-
cipitates then must consist of basic carbonates, such as Cu(OH) 2.
CuCO 3. This shows that the basic character of the hydroxides is
weak, but not as weak as that of A1(OH) 3 , because not even a basic
carbonate of aluminum can be formed. Since no effervescence
took place with the cadmium and silver salts, the precipitates
must have been the neutral carbonates, CdCO 3 and Ag 2 CO 3 , which
indicates that cadmium and silver oxides are more strongly basic
than the others.
(6) Heat a little dry basic copper carbonate by shaking it
in a test tube at some distance above a small flame. The
light blue powder is quickly changed to black, and the seeth-
ing of the dry powder shows that a gas is being expelled.
A drop of lime water is clouded by the gas. To the residue
after it has cooled add 5 cc. of water and then a little HC1 and
note that the black powder dissolves without effervescence.
The ease with which copper carbonate is decomposed by heat
shows further the weakness of the basic character of copper oxide.