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Application of Silicon Compounds

Glass is a non-crystalline transparent super cooled mixture of silicates, one of which is always an alkali metal. It is not a true solid; it has no fixed melting point. Its composition varies with different varieties of glass. Ordinary glass is a mixture of sodium and calcium silicates and may be represented as Na2SiO3 CaSiO3 4SiO2. It is obtained by heating a mixture of sodium carbonate, calcium carbonate (or calcium oxide) and sand (silicon dioxide, SiO2) in a furnace at 1700-1800 K.

The reactions taking place are
Na2CO3 + SiO2 Na2SiO3 + CO2
CaCO3 + SiO2 CaSiO3 + CO2

Glass is hard but brittle in nature. On heating it gradually softens and is converted into a viscous liquid. Changing the raw materials produces many types of glass. Coloured glasses are obtained by adding metal compounds to the glass mix. Small amounts of Co(II), Cr(III), Fe(III) and Mn(IV) compounds impart blue, green, brown and violet colours to glass. Hard glass is obtained by substituting K2CO3 in place of Na2CO3 in the glass mix.
Flint glass is a lead-potash-lime glass and is used for making optical devices. It has a high refractive index. Pyrex glass is obtained when borax or boric acid is added to the glass mix. It is a heat-resistant borosilicate glass and has a very low coefficient of thermal expansion. As it can withstand sudden changes of temperature, it is extensively used for making laboratory apparatus.

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