XII Chemistry BIEK Topic - Glass (Short Note)

Glass (Short Note)
Definition
Glass is a hard material that is usually transparent made by cooling certain molten materials in such a manner that they do not crystallize but remain in amorphous state.

Glass is considered to be a super cooled liquid, i.e. the solid in which the molecules are present as aggregates as in liquid, and are not present in any definite pattern.




Preparation of Glass
The principle ingredients of all types of glass are sand or silica (SiO2). Therefore, glass is one of the most important artificial silicates. For example, the ordinary soft glass or Soda Glass is a mixture of Sodium Silicate (Na2SiO3) and Calsium Silicate (CaSiO3).

Soda glass is manufactured by heating sand (SiO2), sodium carbonate (Na2CO3) and calsium carbonate (CaCO3) in a furnace at high temperature, i.e. 1400ºC




Types of Glass
Glass is classified in a number of ways on the basis of its chemical composition, properties, manufacturing process or it's use. Some important types of glass are as follows.


1. Soft Glass or Soda Glass

Soft glass or Soda glass is an ordinary glass which is a mixture of sodium silicate and sodium calcium silicate. it is also known as window glass.


2. Refractory Potassium Glass

It is a mixture of potassium silicate and calcium silicate. It has high refractive index. This glass is used for making prism, lenses and decorative glass wear.
3. Pyrex Glass

It is boro silicated glass. The main constituents of Pyrex Glass are boroxide (B2O3) and silica (SiO2). This glass has no chemical durability and is soluble even in water.


4. Water Glass

Water glass is just a sodium silicate, which is prepared by the reaction of sodium oxide (Na2O) and silica (SiO2). This glass has no chemical durability and is soluble even in water.


5. Coloured Glass

Coloured glass is prepared by adding certain transition metal oxides. For example copper oxide (CuO) gives light blue coloured glass, where as cobalt oxide (CoO) gives dark blue colour, chromium oxide (Cr2O3) gives green colour and zinc oxide (ZnO) give red coloured glass.


6. Photochromic Glass or Photosensitive Glass

Photochromic glass produces darkness on exposure to bright sunlight but becomes clear again in absence of light. This glass contains silver chloride or silver bromide salts which is sensitive to light, in presence of light, the salt is decomposed to give finely divided black silver, in absence of sunlight, silver and chloride recombine to reform AgCl. This glass is used in sunglasses.


7. Optical Fibres

Optical fibres are thin fibres of silica glass of high purity. They have excellent optical transparency. This glass is used to transmit T.V Programs, Telephone conversion, Computer output etc. It is also used to make a design on glass. This process is called Etching