EXPERIMENTS 237- Hydrolysis of Salts. Recall the fact, or observe by experi-
ment, that the salts HgNO 3 , Hg(NO 3 )2, and ZnCl 2 do not dissolve
in pure water to give a clear solution but that a flocculent residue
of basic salt, HgN0 3 -Hg0H, HgOHNO 3 , ZnOHCl, is left. The
formation of the basic salt is a partial hydrolysis, it leaves the
solution faintly acidic; that the hydrolysis is not extensive, is
shown by the fact that a moderate amount of the corresponding
acid will in each case prevent the formation and precipitation of
the basic salt. - Hydroxides. In separate test tubes place (a) 1 cc. of
12V CuSO 4 ; (6) 10 cc. of 0.1 2V AgNO 3 ; (c) 1 cc. of 12V ZnSO 4 ;
(d) 1 cc. of 12V CdCl 2 ; (e) 5 cc. of 0.12V HgNO 3 ; and (/) 5 cc.
of 0.22V Hg(NO 3 ) 2. To each tube add water to make a volume
of 10 cc. and then add 62V NaOH, 1 drop at a time, shaking
after each drop. Finally add in all 5 cc. of the NaOH.
In every case a precipitate is formed with a small amount of the
reagent as follows: (a) light blue Cu(0H) 2 , (6) brown Ag 2 O,
(c) white Zn(0H) 2 , (d) white Cd(OH) 2 , (e) black Hg 2 O, (/) yellow
HgO. Those precipitates whose formulas are given as hydroxides
are in fact likely to come down as rather indefinite basic salts,
that is, as mixtures of the hydroxides and the salt, but for the sake
of simplicity in discussion it is allowable to regard them as hydrox-
ides. Silver, mercurous, and mercuric hydroxides do not exist in
the solid state; they lose water and form the oxides.
Of the above precipitates, zinc hydroxide alone dissolves freely
in excess of the reagent. It forms the soluble salt sodium zincate,
Na 2 Zn0 2 , and it is thus an amphoteric substance like aluminum
hydroxide (compare Experiments 2, 3, 4, and 5, page 217).
The copper hydroxide does not dissolve altogether in the excess
of the reagent, but the solution acquires a deep blue color which is
seen better after the precipitate settles out. This color shows that
there must be copper in solution, and that the copper hydroxide
possesses amphoteric properties to a slight degree.
- Basic Strength of Silver Oxide. Collect on a filter the
silver oxide precipitate obtained in (6) of the last experiment.
Wash it thoroughly with hot water. Put part of the moist
residue on a piece of red litmus paper and note that the paper
is turned blue. Pour 2 cc. of water over the rest of the brown