Man's physical universe

xanabras

OXYGEN AND SULFUR, CARBON AND SILICON 631

of granite, sandstone, shale, clay, gneiss, and marl are silicates. The

only important rocks not siliceous in nature are limestone, CaCOs, and

dolomite, MgCOs-

Silicon can be obtained in the form of crystals which resemble the

diamond by the reduction of silicon dioxide with aluminum.

Silicon will react with sodium hydroxide to form soluble silicates

and hydrogen. This reaction was used in the World War of 1914-1918

to produce hydrogen for observation balloons.

Silicon Is a Member of the Carbon Family.

Silicon forms many compounds which are similar to the corresponding

carbon compounds.

CO CH4 CCI4 CO2 NasCOs CHCI3

SiO SiH4 SiCl4 Si02 NaaSiOa SiHCU

For example, SiH4 is a colorless, combustible gas which resembles

methane, CH4.

Silicon combines with chlorine to form silicon tetrachloride, SiCU,

which hydrolyzes in contact with the moisture in the air to form an

oxide or hydroxide of silicon. This reaction has been used to produce

smoke screens for warfare.

Silicon dioxide (silica) occurs in such forms as quartz, sand, chalcedony,

amethyst, onyx, jasper, opal, agate, and flint.

Fused silica is resistant to chemical action and has a very small

coefficient of expansion. Silica has such a very high melting-point

that it is difhcult to fashion utensils from it. Some forms of " Pyrex "

glass are nearly pure silica.

In addition to its use in the preparation of

Portland cement and ceramic materials, discussed in Section 4 of this

Unit, and its use to prepare carborundum, silica is chiefly used to

prepare water glass and silica gel.

Water glass is a mixture of soluble silicates, which may be prepared

by any one of the following reactions:

Na2C03 +

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