Modern inorganic chemistry

186 GROUP IV

there is evidence for SiO in the gaseous state. On cooling, a brown

powder is obtained which rapidly disproportionates:

2SiO -> Si + SiO,.

SILICON DIOXIDE, SiO 2

Silica is found naturally in several crystalline forms (e.g. quartz,

tridyniite. cristobalite) and as kieselguhr. a hydrated amorphous

solid possessing great absorptive powers. It is not appropriate to

refer to this oxide of silicon as a dioxide, since, in its crystalline forms,

it forms %giant molecules' in which each silicon atom is linked tetra-

hedrally to four oxygen atoms: the structure can be represented

diagrammatically thus, the linkages extending three-dimensionally:

O

O

-Si-

O

— Si

O

i— O- -Si-

O O

O O O

^Si—O—Si—O —Si

Pure silica may be obtained by hydrolysing silicon tetrafluoride

or the tetrachloride (see the reactions above). When so prepared,

silica is hydrated ; it appears in fact as a gel i.e. a colloidal system

in which a liquid is dispersed in a solid. This gel when filtered off

and dried, loses much of its water, and on heating can be made

anhydrous ; but formation of a solid gel takes place again when the

anhydrous solid is exposed to a moist atmosphere, i.e. the solid

absorbs water. Hence silica gel is a most useful drying agent, for

it has a high capacity for absorbing water and it is also chemically

inactive. Silica is attacked only by hydrofluoric acid, and by alkali

to give silicates :

SiO 2 + 2OH - SiO HO

When silica is fused, silica glass is formed. This has advantages
over ordinary glass in that it is much less easily fused (it softens
at about 1800 K), and has a very low coefficient of expansion. It is,
therefore, used for crucibles and other articles required to be infusible