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Glass, Chemically glass is a mixture of a number of silicates and hence no definite chemical formula, can be assigned to it. However the approximate composition of glass is represented as x M 2O,, y M’O, 6SiO2 where M is a monovalent metal like Na, K, etc and M’ is a bivalent metal like, Ca, Pb, Zn etc. x and y are whole numbers. Thus ordinary glass may be represented as Na 2O,, CaO, 6SiO2., Glass is an amorphous, hard, brittle transparent super cooled liquid of infinite viscosity, obtained by fusing a mixture of sodium carbonate, calcium carbonate and silica (sand)., General properties of glass, 1. Glass is amorphous, 2. It can be moulded into desired shape., 3. It is affected by alkalis, 4. It is not affected by air, water, or acids, 5. It is very brittle, 6. It has no definite melting point, 7. It is a good electrical insulator, 8. It can absorb, reflect, or transmit light, 9. On heating it softens, Types of Glasses, 1. Soda glass, Soda glass is the ordinary glass made by sodium and calcium silicates. Row materials, required for the production are sand, limestone and sodium carbonate. It softens easily, on heating. Soda glass is the cheapest and the most common type of glass. The major, disadvantage of soda glass is that it is brittle and hence breaks easily. The soda glass is, used for making jars, dishes, bottles, mirrors, window panes, and light bulbs., 2. Borosilicate glass (Pyrex glass or Heat Resistant glass), – Pyrex glass is made up by fusing a mixture of sodium carbonate, potassium carbonate,, silica, borax and aluminium oxide. It is also known as borosilicate glass because it is, chemically sodium aluminium borosilicate. It has a very low coefficient of expansion,, so the laboratory apparatus made of Pyrex glass can be heated to high temperatures, without the risk of breaking. Pyrex glass is more resistant to chemicals and much

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stronger than ordinary glass. It is used for making laboratory apparatus like flasks,, beakers, etc., which are to be used at high temperatures. Pyrex glass is also used for, making pharmaceutical containers, ovenware, television tubes etc., 3. Safety glass, – Safety glass is prepared by placing a thin sheet of transparent plastic between two, layers of glass by applying a suitable adhesive on its both sides. The three layers are, then joined together by the action of heat and pressure to form a single sheet of safety, glass. Safety glass is very strong. It does not break easily under ordinary impact. When, hit by a heavy object, safety glass cracks but does not break into small pieces. Safety, glass is used for making windscreens in automobiles. It is also used for making bullet, proof glass., 4. Insulating glass, It is a transparent unit of glass, prepared by two or more plates of glass separated by 6, to 13 mm gap filled with dehydrated air and sealing around the edges. This air gives, insulation against heat. If such a glass is used for separating rooms, the apartment, remains cool during summer and warm during winter.