NCERT · Class 10 · Science · Metals and Non-metals(a) Differentiate between roasting and calcination with balanced chemical equations using zinc ores as examples. (b) Explain the process of electrolytic refining of copper with a clear description of the setup, electrodes used, and the chemical reactions involved at both electrodes.

Part (a): Difference Between Roasting and Calcination

1. Roasting:

• Definition: Roasting is the process in which a sulphide ore is strongly heated in the presence of excess air to convert it into a metal oxide.
• Example (Zinc Sulphide / Zinc Blende): $$2ZnS(s) + 3O_2(g) \xrightarrow{\Delta} 2ZnO(s) + 2SO_2(g)$$
• Gaseous By-product: Sulfur dioxide ($SO_2$) gas is released.

2. Calcination:

• Definition: Calcination is the process in which a carbonate ore is strongly heated in the absence or limited supply of air to convert it into a metal oxide.
• Example (Zinc Carbonate / Calamine): $$ZnCO_3(s) \xrightarrow{\Delta} ZnO(s) + CO_2(g)$$
• Gaseous By-product: Carbon dioxide ($CO_2$) gas is released.

3. Reduction of Oxide to Metal:

• The obtained zinc oxide ($ZnO$) is subsequently reduced to metallic zinc using a reducing agent such as carbon (coke): $$ZnO(s) + C(s) \xrightarrow{\Delta} Zn(s) + CO(g)$$

Part (b): Electrolytic Refining of Copper

\nElectrolytic refining is the process used for purifying crude metals obtained after extraction.

1. Experimental Setup:

• Anode (Positive Electrode): A thick block of crude or impure copper metal.
• Cathode (Negative Electrode): A thin strip of pure copper metal.
• Electrolyte: An aqueous solution of acidified copper sulphate ($CuSO_4$).

2. Working Mechanism:

• When an electric current is passed through the electrolyte, pure copper metal from the impure anode dissolves into the electrolyte solution.
• Simultaneously, an equivalent amount of pure copper ions from the electrolyte gets deposited on the cathode.

3. Chemical Reactions at Electrodes:

• At Anode (Oxidation): Impure copper loses electrons and dissolves into the solution as copper ions. $$Cu(s) \text{ (impure)} \rightarrow Cu^{2+}(aq) + 2e^-$$
• At Cathode (Reduction): Copper ions from the electrolyte solution gain electrons and get deposited as pure copper metal. $$Cu^{2+}(aq) + 2e^- \rightarrow Cu(s) \text{ (pure)}$$

4. Anode Mud:

• Soluble impurities present in the crude copper dissolve into the electrolyte solution.
• Insoluble impurities (such as precious metals like gold and silver) settle down at the bottom of the anode tank, which is known as anode mud.