The use of silica to produce glass-like materials in decorative
items dates back as far as 1500 B.C. By the end of the 18th century, chemists
understood that there was some sort of common link between sand, quartz, and silica.
In 1824, by heating chips of potassium metal in a container of silica (silicon
tetrafluoride), silicon was produced in a mixture of potassium fluoride. When
the potassium fluoride was chemically separated, silicon was isolated and identified
in a fairly high-quality sample.
Silicon is the most abundant solid element, being second only to oxygen. Silicon
makes up more than 25% of the earth's crust. It is a semimetal,
or metalloid. It rarely occurs in
elemental form, virtually all of it existing as compounds. Silicon is best known
in two forms:
a dark brown, powdery amorphous form, which is best known for its presence
as beach sand.
a grey, metallic-looking crystalline form, which is best known for its impact
upon modern civilization - as the foundation material for electronic semiconductor
Other commercial applications of silicon include:
Calcium and sodium silicates, Na2SiO3.CaSiO3.3SiO2,
or soda glass that is used in ordinary glasses, bottles and windows. It
is made by fusing sand (SiO2), sodium carbonate and limestone
(CaCO3) at about 1400oC.
Pyrex glass, which is used for laboratory equipment and ovenware, has
a higher softening temperature and is resistant to sudden changes of temperature
because it has a smaller thermal coefficient of expansion. In this type
of glass the proportion of CaSiO3 is lower and some silica
is replaced by boron trioxide, B2O3.
Coloured glass has small amounts of other ions such as:
Crystal glass used for high grade tableware and chandeliers contains
some lead oxide. This glass has a higher refractive
index than ordinary glass. This causes it to "sparkle" since the pieces
are cut at such angles that total internal reflection and spectral dispersion
of light occur.
Sodium silicate, Na2SiO3, which is known as water
glass. It is a silicon compound that dissolves in water. It is produced
by reacting silicon dioxide with sodium hydroxide at a high temperature. The
thick clear liquid which results has a wide range of viscosities according
to water content. The major uses of water glass are as a dispersing agent
for clay suspensions, as a fire-retardant coating for paper and fibreboard,
and as a detergent, soap and adhesive builder. Silica gel is a porous solid
material made by treating sodium silicate with sulfuric acid. It is used as
a dehumidifier or a dessicant.
Aluminosilicate minerals or zeolite,
which exhibit ion-exchange properties are
used for water softening.
Silicon carbide, SiC or carborundum, which is a tough, abrasive substance
that is used for grinding metals. It is a dark, bluish-black crystal that
is produced by heating sand and coke (carbon) together in
an electric furnace.
Silicon tetrachloride, SiCl4, which is used by the military for
generating smoke screens from dense clouds of white vapour.
Silicon tetrafluoride, SiF4, is a colourless, choking gas that
is mainly used in production of other fluorosilicates.
Portland cement, which is a complex
mixture of calcium silicates and aluminosilicates.
Silicones are polymeric synthetic silcon compounds. Silicones are used as
lubricants and water repellents and in waxes and varnishes. Silicone rubbers
are superior to natural rubbers in their resistance to both high and low temperatures