For manufacturing of glass, which among the following is used as a source of silica ?

The correct answer is D, Sand. Clean sand is made almost entirely of quartz, which is silica or silicon dioxide, and silica is the main ingredient of glass. Ordinary soda-lime glass is made by melting a mixture of sand, soda ash (sodium carbonate) and limestone (calcium carbonate) in a furnace at around 1,500 degrees Celsius, then cooling the melt quickly so it sets without forming crystals. Soda ash lowers the melting point of silica and limestone makes the glass resistant to water. Adding other substances gives special glasses: boron oxide gives heat-resistant borosilicate glass used in laboratories, lead oxide gives sparkling crystal glass, and metal oxides give colour, such as cobalt for blue. Option A is wrong because clay is mostly aluminium silicate and is used for bricks and pottery, not as the silica source for glass. Option B is wrong because wood powder is organic matter that would simply burn in the furnace. Option C is wrong because coconut shell is also organic and is used to make activated charcoal. Exam tip: glass = sand + soda ash + limestone; silica comes from sand.