GROUP IV 193hydrated oxide obtained when metallic tin reacts with concentrated
nitric acid:
Sn + 4HNO 3 -> SnO 2 i + 4NO 2 t + 2H 2 OTin(IV) oxide is insoluble in water, but if fused with sodium hy-
droxide and the mass extracted with water, sodium hexahydroxo-
stannate(IV) is formed in solution:
SnO 2 4- 2NaOH + 2H 2 O -> Na 2 [Sn(OH) 6 ]If a dilute acid is added to this solution, a white gelatinous precipitate
of the hydrated tin(IV) oxide is obtained. It was once thought that
this was an acid and several formulae were suggested. However, it
now seems likely that all these are different forms of the hydrated
oxide, the differences arising from differences in particle size and
degree of hydration. When some varieties of the hydrated tin(IY)
oxide 'dissolve' in hydrochloric acid, this is really a breaking up of
the particles to form a colloidal solution—a phenomenon known
as peptisation.
Oxides of lead
LEAD(II) OXIDE, PbO
Lead(II) oxide is the most stable oxide of lead; it exists in two
crystalline forms. One form is reddish yellow in colour, with a
tetragonal lattice, and is called litharge. The other form, yellow in
colour, has a rather greater density and a rhombic lattice ; it is called
massicot. Litharge is obtained when molten lead is oxidised by a
blast of air. By more careful heating, or by heating lead carbonate
or lead nitrate, massicot is obtained. Litharge is the stable form at
room temperature, but massicot changes only very slowly to
litharge under ordinary conditions.
Lead(II) oxide is the most basic oxide formed by a Group IV
element. It dissolves easily in acids to give lead(II) salts but it also
dissolves slowly in alkalis to give hydroxoplumbates(II) and must,
therefore, be classed as an amphoteric oxide, for example :
2H+ ->Pb^2 + + H 2 O
PbO + 4OH~ + H 2 O -> [Pb(OH) 6 ]^4 ~Lead(II) oxide is easily reduced to the metal when heated with a
reducing agent such as hydrogen, carbon or carbon monoxide, for
example :
PbO + H 2 -> Pb 4- H 2 O