GROUP IV 187
and to resist chemical attack. It is also used for certain optical plates
and lenses, since it transmits ultra-violet light better than ordinary
glass.'SILICIC ACID" AND THE SILICATES
When acid is added to any soluble silicate, the following reaction
occurs:
SiO^(aq) + 2H+ ^ H 2 SiO 3 (aq) -> SiO 2 .xH 2 O
and the 'silicic acid' is converted to insoluble, hydra ted silica similar
to that already described.
A soluble silicate—a trioxosilicate—is obtained when silica is
fused with sodium carbonate:
SiO 2 + Na 2 CO 3 -> Na 2 SiO 3 + CO 2 t
This is an acid-base reaction, in which the base is the oxide ion O^2 ~
(p. 89); the acidic oxide SiO 2 displaces the weaker acidic oxide
CO 2 in the fused mixture. But in aqueous solution, where the O^2 ~
ion cannot function as a strong base (p. 89), carbon dioxide displaces
silica, which, therefore, precipitates when the gas is passed through
the aqueous silicate solution. In a fused mixture of silica and a
nitrate or phosphate, the silica again displaces the weaker acidic
oxides N 2 O 5 and P 4 O 10 :
4KNO 3 4- 2SiO 2 -» 2K 2 SiO 3 + 2N 2 O 5
2N 2 O 5 -> 4NO 2 4- O 2
2Ca 3 (PO 4 ) 2 + 6SiO 2 ~» 6CaSiO 3 -f- P 4 O 10This latter reaction is used in the extraction of phosphorus (p. 208).
The product of the fusion of silica with sodium carbonate, sodium
silicate (strictly called sodium polytrioxosilicate but usually meta-
silicate), dissolves in water to give a clear, viscous solution known as
'waterglass'. It hydrolyses slowly and silica is precipitated. Besides
the metasilicate, other silicates of sodium are known, e.g. the poly-
tetroxosilicate (orthosilicate), Na 4 SiO 4. Only the silicates of the
alkali metals are soluble in water. Other silicates, many of which
occur naturally, are insoluble, and in these substances the poly-
silicate anions can have highly complicated structures, all of which
are constructed from a unit of one silicon and four oxygen atoms
arranged tetrahedrally (cf. the structure of silica). Some of these
contain aluminium (the aluminatesilicates) and some have import
ant properties and uses.