question_answer

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

Answer:

  Sol.         (a) Roasting It is the process in which sulphide ores of the metals are converted into oxides by heating them in the presence of excess air. For example, zinc sulphide is converted into zinc oxide by roasting. Calcination It is the process in which carbonate ores of the metals are decomposed into oxides by heating them in the absence or limited air. For example, zinc carbonate is decomposed into zinc oxide and carbon dioxide by calcination.                                                       (2) Out of roasting and calcination, only roasting is used for sulphide ores. This is because it is easier to obtain metal from its oxide as compared to its sulphide. (b) (2) (c) Anode - Impure copper Cathode - Strip of pure copper Electrolyte - Acidified copper sulphate solution.                                                             (1)