(a) Distinguish between 'roasting' and 'calcination'. Which of the two is used for sulphide ores and why ?
(b) Write a chemical equation to illustrate the use of aluminium for joining craked railway lines.
(c ) Name the anode, the cathode and the electrolyte used in the electrolytic refining of impure copper.

Explanation

(a) For distinction between calcination and roasting.
For the sulphide ores, the process of roasting is commonly used. It is carried by heating the ore helow its melting point with excess of air. As a result, the sulphide ore is converted to its oxide form. For example,
`underset(("Zinc blende"))(2ZnS)+ 3O_(2)rarr2ZnO + 2SO_(2)(g)`
`underset(("Galena"))(2PbS) + 3O_(2) rarr2PbO + 2SO_(2)(g)`
(b) Cracked railway lines can be joined or welded with the help of thermit welding. Thermit is a mixture of ferric oxide and aluminium powder. The chemical equation involved is :
`2Al + Fe_(2)O_(3) rarr Al_(2)O_(3) + underset(("Molten"))(2Fe) + "heat (exothermic)"`
For the details of the process.
(c ) In the electrolytic refining of copper ,
Anode : A rod of impure copper.
Cathode : A rod of pure copper.
Electrolyte : An aqueous solution of copper sulphate containing a small amount of sulphuric acid.