Glass in Chemistry.
Ordinary glass is solid mixture of silica (SiO2), sodium silicate (Na2SiO3) and calcium silicate (CaSiO3).
Glass is a super cooled liquid hence; it has no definite crystal structure and melting point.
But the average composition of glass is x . M2O . yM’O . 6SiO2, where M is a monovalent alkali metal like Na, K etc., M’ is a bivalent metal like Ca, Pb, Zn etc. and x and y are whole numbers.
Thus, ordinary glass may be represented as Na2O . CaO . 6SiO2.
Hence we can say that glass is a mixture, not a compound.
Annealing of Glass
The process of slowly cooling of glass in annealing kiln is called Annealing of glass.
Types of Glass
Glass is of following Types :
1. Soft – Glass :
It is sodalime silicate glass (Na2O . CaO . 6SiO2).
It melts at low temperature It is used in manufacturing of bottles, test tubes and glass of windows etc.
2. Hard – Glass :
It is potash lime silicate (K2O . CaO . 6SiO2).
It melts at high temperature in comparison to soft glass and is used in manufacturing of flask, etc.
3. Flint – Glass :
It is lead – potash silicate (K2O . PbO . 6SiO2) and is used in manufacturing of prism and lens of optical instruments.
4. Crookes – Glass :
It is special type of optical glass containing circum oxide which cut off ultra violet rays harmful to eyes and used in manufacturing of lens of spectales.
5. Pyrex – Glass :
It is a mixture of sodium aluminum borosilicates (Na2O . Al2O3 [B2(SiO3)3]). Its coefficient of expansion is very low and hence it can withstand sudden temp changes.
It has high percentage of silica, about 80%. It is used in manufacture of high quality equipments in laboratory because it does not melt at very high temperature.
6. Quartz – Glass :
It is obtained from pure silica. It has a low coefficient of expansion and does not break even when plunged in water while red hot.
7. Ground Glass : It is prepared by grinding ordinary sand (soft) glass by emery and turpentine oil.
8. Reinforced Glass : It has a network of wires embedded in and does not shatter easily.
9. Safety – Glass :
It is also known as Shatter Proof glass. It is prepared by placing a layer of transparent plastic glass (usually a sheet of vinyl acetate resin) between two layers of glass by means of a suitable adhesive.
This glass does not break easily under ordinary impact. It is used in making wind screen of automobiles, aero – planes, trains etc.
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