Read the following and answer the questions that follow:
In a school laboratory, a teacher demonstrated the extraction of metals using different methods. For metal X (found as its sulphide ore), she first heated it strongly in excess air, then reduced the product with carbon to obtain the metal. For metal Y, she showed that simply heating its oxide in a test tube was sufficient to obtain the metal as droplets. For metal Z (a very reactive metal), she performed electrolysis of its molten chloride to deposit it at one of the electrodes.
(i) What is the name of the process used to convert the sulphide ore of metal X into its oxide? Write a general equation for this conversion. (1 mark)
(ii) What does the extraction of metal Y by heating alone tell you about its position in the activity series? Name one such metal. (1 mark)
(iii) At which electrode is metal Z deposited during electrolysis? Write the half-reaction that occurs at that electrode. (1 mark)
(iv) During the electrolytic refining of a metal, an impure residue settles at the bottom of the electrolytic cell. What is this residue called, and what does it contain? (1 mark)
Generated by claude-sonnet-4-6 · 2026-06-26 01:06 · grounding rag
Model Answer
(i) The process is called roasting. The sulphide ore is heated strongly in excess air to convert it into a metal oxide.
General equation: $2\text{MS} + 3\text{O}_2 \xrightarrow{\Delta} 2\text{MO} + 2\text{SO}_2$
(ii) It tells us that metal Y is low in the activity series (least reactive). Its oxide is unstable and is easily reduced by heat alone. Mercury (Hg) is one such metal.
(iii) Metal Z is deposited at the cathode (negatively charged electrode).
Half-reaction at cathode: $\text{Na}^+ + e^- \rightarrow \text{Na}$
(iv) The insoluble residue that settles at the bottom is called anode mud. It contains insoluble impurities from the impure metal, which may include precious metals like gold and silver.
Source: Chapter 3, Sections 3.4.3, 3.4.4, 3.4.5, 3.4.6
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Explanation
- (i) Roasting = heating sulphide ore in excess air → metal oxide + SO₂. Don't confuse with calcination (carbonate ores, limited air).
- (ii) Key link: metals low in activity series → oxide easily decomposed by heat alone. Mercury is the textbook example (from cinnabar/HgS).
- (iii) Cathode is negative → attracts positive metal ions → metal deposited. The half-reaction given in the textbook uses Na⁺ (as metal Z is a very reactive metal extracted by electrolysis of molten chloride — fits Na/Mg/Ca).
- (iv) "Anode mud" is the exact CBSE term. Examiners expect both the name and that it contains insoluble/precious metal impurities.