31 Year NEET Previous Year Questions: General Principles & Processes of Isolation of Elements (Old NCERT) - NEET MCQ

Si obtained by reduction of SiCl₄ with H₂ is further purified by zone refining method to get Si of very high purity. Silicon is purified by zone-refining process because the impurities present in it are more soluble in the liquid phase than in the solid phase.

Zone refining is based on the difference in solubility of impurities in molten and solid state of the metal. This method is used for obtaining metals of very high purity.

The sulphide ore is roasted to oxide before reduction because the ΔG^o_f of most of the sulphides are greater than those of CS₂ and H₂S, therefore neither C nor H can reduce metal sulphide to metal. Further, the standard free energies of formation of oxide are much less than those of SO₂. Hence oxidation of metal sulphides to metal oxide is thermodynamically favourable.

Leaching is the selective dissolution of the desired mineral leaving behind the impurities in a suitable dissolving agent eg Argentitie or Silver glance, Ag₂S is an ore of silver. Silver is extracted from argentite by the mac-Arthur and  Forest process (leaching process).

Pigironor castiron contains 3 – 5% carbon and varying amounts of Mn, Si, P and S which makes the iron hard and brittle.

Zr and Ti are purified by van Arkel method.

In blast furnace at about 1270 K, calcium carbonate is almost completely decomposed to give CaO which acts as a flux and combines with SiO₂ present as impurity (gangue) in the ore to form calcium silicate (fusible slag)

[Fe₃O₄ ⇒ Magnetite]
[CuCO₃ Cu(OH)₂ – Malachite]
Pyrolusite – MnO₂ Cassiterite – SnO₂.

Cuprous oxide formed during roasting of cuprous sulphide is mixed with few amount of cuprous sulphide and heated in a reverberatory furnace to get metallic copper.

Invar is a nickel iron alloy, Bell metal is an alloy of about 80% copper and 20% tin, Bronze is also an alloy of copper and tin.
Steel : It always have few % of carbon.

Al₂O₃ cannot be reduced by carbon.