Silicon

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 components.

Other commercial applications of silicon include:

Glass making:
- 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 1400^oC.
- 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:
  - green chromium(III),
  - violet manganese(IV),
  - blue cobalt(II), or
  - brown iron(III).
- 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 and chemicals.

BCIT Chemistry Resource Center
http://nobel.scas.bcit.ca/resource/