296 ELEMENTS OF GROUP IVvisible. Let the rest of the yellow solution grow warm and
note that chlorine gas is evolved and that a crystalline white
precipitate separates.The yellow solution contains lead tetrachloride. It is puzzling
to explain why lead dioxide will not react with two of the strong
acids tried, yet does react with hydrochloric acid to give what is
apparently a salt, PbCU. The explanation lies in the character of
lead tetrachloride, which is practically un-ionized, and therefore
is hardly to be classed as a salt. In the anhydrous condition it is
a liquid like carbon tetrachloride. Furthermore it combines with
excess HC1 to form the complex acid H 2 PbCl 6 , of which the am-
monium salt (NILJ 2 PbCl6 can be crystallized. By comparison,
if nitric acid reacted with lead dioxide, the tetranitrate, Pb(NO 3 )4,
would be the product; this presumably would be highly ionized
like all nitrates, which means that it would have to hydrolyze
completely.
With a large amount of water the lead tetrachloride hydrolyzesPbCU + 4H 2 O -»• Pb (OH) 4 1 + 4HC1giving the brown precipitate. In concentrated solution it decom-
poses into chlorine and PbCl 2 , the white crystalline precipitate.
- Stability of Lead Carbonate, (a) To a neutral lead
nitrate solution add Na 2 CO3 solution drop by drop, noting the
white precipitate and the absence of effervescence.
(6) Heat a little dry white lead carbonate in a test tube
and note that it changes to a yellow powder when it has
become moderately hot. The temperature is much higher
than that required to decompose copper carbonate. Treat
some of the residue with dilute HNO 3 and note that it dis-
solves without effervescence.
The facts that lead carbonate PbCO 3 will precipitate without
hydrolyzing to a basic carbonate, and that lead carbonate must
be heated moderately hot to be decomposed, both confirm the
conclusion, already made, that PbO is a distinctly basic oxide.