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Friday, December 20, 2019

Silicon Oxide : Silica (SiO2):

Silicon is unable to form pp - pp bond with oxygen atom due to its relatively large size. Thus it satisfies its all four valency with four oxygen atoms and constitutes three - dimensional network. In this structure each oxygen atom is shared by two silicon atoms. Three crystalline modification of SiO2 are quartz, cristobalite and tridymite of which quartz and cristobalite are important.
Quartz (rock crystal) is the purest form of silica. It is used in preparation of costly glasses and lenses. It is also used as piezoelectric material (crystal oscillators and transducers).
Several amorphous forms of silica such as silica gel and fumed silica are known. Silica gel in made by acidification of sodium silicate and when dehydrates, is extensively used as a drying agent in chromatographic and catalyst support.


Artificially silica can be obtained by following methods
Pure silica is colourless but sand is brownish or yellowish due to presence of impurities of iron oxide.
Question: Why SiO2 is solid while CO2 is a gas?
Answer: This is because SiO2 exists in silicate form where Si forms tetrahedron SiO42- ions which are regularly attached with each other and form a giant network structure hence remains solid.

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