(i) The metal should form a volatile compound with a suitable reagents and impurities should not.
(ii) The volatile compound should be easily decomposable so that pure metal can be easily obtained.

They are refined by zone refining. It is based on the principle that the impurities are soluble in the melt (molten state) than in the solid-state of metal.

The principal ore of aluminium is bauxite  (Al₂O₃) It contains SiO₂ , iron oxides and titanium oxide as impurities. The significance of leaching in the extraction of aluminium from bauxite is to remove. the impurities from the ore.

The entropy is higher if the metal is in liquid state than when it is in solid state. The value of entropy change (DS) of the reduction process is more on positive side when the metal formed is in liquid state and the metal oxide being reduced is in solid state. Thus the value of DrG V becomes more on negative side and the reduction becomes easier.

(i) Dil. NaCN forms a complex with Ag₂S which on reduction with zinc gives silver metal. Impurities remain unaffected and can be filtered off.
\(Ag_{ 2 }S+4NaC{ N }\rightarrow 2Na[Ag(CN)_{ 2 }]+{ Na }_{ 2 }S\) 
\(Na[Ag(CN)_{ 2 }]+Zn\overset { Zn }{ \rightarrow } Na_{ 2 }[Zn(CN)_{ 4 }]+2Ag\)
(ii) Iodine reacts with Titanium to form Titanium iodide which on heating strongly gives pure titanium and iodine back.
\(Ti(impure)+{ 2I }_{ 2 }\rightarrow { TiI }_{ 4 }\) 
\({ TiI }_{ 4 }\overset { heat }{ \rightarrow } Ti(pure)+{ 2I }_{ 2 }\)
(iii) Cryolite lowers the melting point of alumina and increases electrical conductivity.
In other words, fused \(Al_{ 2 }{ O }_{ 3 }\) is bad conductor of electricity. Therefore, cryolite (Na₃AlF₆) is added to purified \(Al_{ 2 }{ O }_{ 3 }\) which reduces the melting point of \(Al_{ 2 }{ O }_{ 3 }\) mixture around 1140 K and also increases electrical conductance.

(i) Extraction of zinc from zinc oxide:
The reduction of zinc oxide is carried out by coke at 673 K.
The oxide is heated with coke.
\(ZnO+\underset { Coke }{ C } \overset { 673K }{ \rightarrow } Zn+CO\)
Corbon monoxide formed can also reduce ZnO further to zinc.
\(ZnO+CO\overset { heat }{ \rightarrow } Zn+CO_{ 2 }\)
(iii) Titanium (Ti) is purified by van Arkel method. In this method, Ti is heated with iodine vapours at about 870 K to form volatile TiI₄ which is heated over tungsten filament at 2075 K to give pure Ti.
\(\underset { Impure }{ Ti } +{ 2I }_{ 2 }\overset { 870K }{ \rightarrow } { TiI }_{ 4 }\overset { 2075K }{ \underset { W }{ \rightarrow } } \underset { pure }{ Ti } +{ 2I }_{ 2 }\)

(i) It prevents certain sulphides like ZnS to enter the froth in presence of PbS, therefore, helps in their separation. Sodium cyanide is used as depressant in the separation of ZnS from PbS.
(ii) It acts as flux.
(iii) Graphite rods act as anode in the extraction of aluminium.

Aluminium vessels are not affected either by acids or alkalies or by other chemicals, but copper and bronze vessels react with these forming poisonous chemical compounds.That's why, vessels are made up of aluminium are preferred.

(i) Aluminium metal
(ii) Aluminium vessels are not affected either by acids or alkalies or by other chemicals, but copper and bronze vessels react with these forming poisonous chemical compounds.That's why, vessels are made up of aluminium are preferred.
(iii) Aluminium is very useful metal. It is very light and good conductor of both heat and electricity. Although it is placed high up in the series but layer of its oxide deposited on the surface of the metal checks its corrosion.

Keen observation, love for mankind, love for enviroment, will to explore and experiment, general awareness.

Only sulphide ores like ZnS, CuFeS₂, etc. are concentrated by froth floatation process