NDA (I) 2023 General Ability Test · Question 111 of 149

Correct — D, Sand. Ordinary glass is made mainly from silica (silicon dioxide, SiO2), and the cheap, abundant source of that silica is sand — specifically silica or quartz sand. So sand supplies the silica needed for glass manufacture.

• (a)Fine clay soil — Clay is an aluminosilicate used to make bricks, pottery and ceramics, not the silica source for glass.
• (b)Wood powder — Wood powder is organic and contains no silica; it would burn away rather than form glass.
• (c)Coconut shell — Coconut shell is organic matter (used to make activated carbon), not a source of silica.

Glass is an amorphous solid whose chief raw material is silica. In making ordinary soda-lime glass, silica sand is melted together with soda ash and limestone. Sand provides the silica network, soda ash lowers the melting temperature and limestone makes the glass durable.

Three options are organic or clay-based materials that supply no silica; only sand is essentially silicon dioxide. Recognising that silica means silicon dioxide, and that sand is largely quartz (SiO2), makes the choice clear.

• The chief raw material of glass is silica (SiO2), supplied by sand.
• Ordinary soda-lime glass is made from sand, soda ash (Na2CO3) and limestone (CaCO3).
• Silica has a very high melting point, and soda ash acts as a flux to lower it.
• Glass is an amorphous solid, so it softens over a range rather than having a sharp melting point.

• Confusing clay (used for ceramics) with sand (used for glass).
• Thinking organic materials like wood or coconut shell supply silica.

NDA/UPSC ask for the raw materials of glass or which is not correct about glass — remember sand gives the silica, with soda ash and limestone added.

No directly related past PYQ was found.

• practice — not a real PYQ

  The main raw materials for making ordinary soda-lime glass are

  • (a)clay, feldspar and quartz
  • (b)sand, soda ash and limestone
  • (c)gypsum, coke and iron ore
  • (d)bauxite, cryolite and carbon

  Answer(b) sand, soda ash and limestone — sand supplies silica, soda ash lowers the melting point and limestone adds durability.

• practice — not a real PYQ

  Silica used in glass manufacture is chemically

  • (a)calcium carbonate
  • (b)sodium carbonate
  • (c)silicon dioxide
  • (d)aluminium oxide

  Answer(c) silicon dioxide — silica is SiO2, the chief ingredient of glass, obtained from sand.