Glass: characteristics and types

• It is a metallic silicate of various metals.
• It is a hard, transparent, amorphous and super-cooled liquid.
• It is an important industrial material which is used in our daily life.
• It is prepared by heating a mixture of silica, sodium carbonate and calcium carbonate in a furnace.

Characteristics

• It is a hard, transparent homogenous mixture of various metallic silicates.
• It does not have a fixed composition and particular formula.
• It does not have a fixed melting point.
• The properties of glass depend on the chemicals added during its manufacture.

Types of glass

• Glass is of following types depending on its composition.

a) Quartz glass

• It is a pure crystalline glass which is prepared by heating silica at about 1600⁰C and then cooling the product.
• It is also called silica glass.
• It is very strong and is insoluble in water and most of the acids.
• It is not affected by chemical reagents and has very low coefficient of thermal expansion.
• So, when red hot quartz glass is kept in cold water, it does not crack.
• It is used for making electrical devices, laboratory appliances like crucibles, basins, etc. and colorful gems.

Pure silica (SiO₂)   1600⁰C   →    Quartz glass

b) Water glass

• Sodium silicate, or potassium silicate, is called water glass.
• It is prepared by heating silica with sodium carbonate or potassium carbonate at about 800⁰c and cooling the product.
• It is easily soluble in water.
• So, it is called water glass. It is fire proof.
• It is used for making fireproof materials, silica garden, gums, glue and paste.

Na₂CO₃ + SiO₂     800⁰C →  Na₂O∙SiO₂ + CO₂

SiO₂+ K₂CO₃ (800⁰C) → K₂SiO₃ + CO₂

c) Ordinary glass or soda-lime glass

• The homogeneous mixture of sodium silicate and calcium silicate is called ordinary glass.
• It is also called soft glass as it melts at low temperature.
• This glass is prepared by heating a mixture of 50%silica, 25% pieces of glass, 15% sodium carbonate and 10% calcium carbonate at about 1500⁰C in a tank furnace.
• In ordinary glass, limestone helps to make the solution insoluble and glass pieces help in melting.
• When lead monoxide is added during the manufacture of ordinary glass, it becomes lead silicate glass. Lead monoxide increases the refractive index of glass and decreases the transparency.
• So, it is used for making prisms, lenses, etc.
• It is used in manufacture of simple bottles, window panes, glass sheets, laboratory apparatus, etc.

2SiO₂ +Na₂CO₃ + CaCO₃  (1500⁰C)   →  Na₂SiO₃.CaSiO₃ + 2CO₂

d) Hard glass

• It is the homogeneous mixture of potassium silicate and calcium silicate.
• It is obtained by heating a mixture of silica, potassium carbonate and calcium carbonate at a high temperature.
• It is also called potash -lime glass.
• It is hard and does not melt at ordinary temperature.
• It is used for making laboratory apparatus like beakers, hard glass test tube, electric bulb, tube-light, etc.

2SiO₂+K₂CO₃+CaCO_{3    →}    K₂SiO₃.CaSiO₃ + 2CO₂

                                                                             Image source: chemistryabc

e) Borosilicate glass or pyrex glass

• It is a homogeneous mixture of sodium silicate, calcium silicate and boron silicate.
• It is obtained by heating a mixture of silica, sodium carbonate, calcium carbonate and boric oxide at a high temperature.
• The effect of change in temperature is less in Borosilicate glass due to the presence of boron silicate.
• This glass is hard and does not melt at ordinary temperature.
• It is stronger than ordinary glass.
• It is more resistant to heat and chemicals.
• It is used for making laboratory apparatus like test tubes, flasks, beakers, oven wares, ampoules and other pharmaceutical containers.

5SiO₂+ Na₂CO₃+ CaCO₃+ B₂O_{3   →}   Na₂SiO₃.CaSiO₃.B₂(SiO3)₃ + 2CO₂

f) Lead crystal glass

• It is a homogeneous mixture of potassium silicate and lead silicate.
• This glass is also called flute glass or optical glass.
• This glass is prepared by heating a mixture of silica, potassium carbonate and lead monoxide at a high temperature.
• It has a high refractive index due to the presence of lead silicate.
• So, this glass is used for making lenses, prisms, electric bulbs, TV screens, spectacles, expensive drinking glasses, radar tubes, etc.
• It is also used to cut-off UV radiation.

2SiO₂+ K₂CO₃+ PbO →  K₂SiO₃.PbSiO₃ + CO₂

g) Colored glasses

• Simple or ordinary glasses can be made colorful by adding certain metal oxides to the molten mass during the preparation of glass.
• Certain metal oxides impart a particular color to the glass.
• So, the selection of the metal oxide depends on the color of our choice.
• These are used for making sunglasses, disco light signals, artificial gems, etc.
• Some of the metal oxides and the color imparted by them are given below.

References: 

i) https://www.jagranjosh.com/general-knowledge/glass-composition-types-and-uses-1456986737-1

ii) https://www.britannica.com/technology/glass

Glass: characteristics and types