Extraction of Metals – Long Answer Questions (CBSE Class 10 Science)
Medium Level (Application & Explanation)
Q1. Explain the steps involved in the extraction of a metal from its ore, using iron as an example.
Answer:
Extraction of iron involves three main steps: concentration of the ore, reduction to crude metal, and refining.
Step 1: Concentration
Haematite (Fe₂O₃) contains iron along with impurities (gangue) such as sand and clay.
The ore is crushed and washed to remove gangue.
Magnetic separation may be used as iron is magnetic.
Step 2: Reduction (Blast Furnace Process)
Concentrated haematite is mixed with coke (carbon) and limestone and put into the blast furnace.
Hot air is blown inside, and coke acts as a reducing agent, removing oxygen from Fe₂O₃ and forming iron:
Fe₂O₃ + 3C → 2Fe + 3CO
Step 3: Refining
The crude iron collected at the bottom of the furnace is impure and is later purified depending on its use (wrought iron, steel).
Each step is essential to remove impurities and obtain pure iron for commercial use.
Q2. Why does the method of extracting a metal depend on its position in the reactivity series? Illustrate with suitable examples.
Answer:
The reactivity series ranks metals from most reactive (potassium) to least reactive (gold).
Highly reactive metals (like sodium, aluminium) form stable compounds; simple heating cannot break these compounds. They require electrolysis to extract the metal.
E.g., Sodium is obtained by electrolyzing molten sodium chloride.
Moderately reactive metals (like iron, zinc) can be obtained by reduction with carbon since their oxides are not so stable.
E.g., Iron is extracted by reducing haematite with carbon in a blast furnace.
Least reactive metals (like gold, silver) are found in native form (pure), and when present as oxides, can be extracted by simple heating.
E.g., Gold is found as nuggets, and silver is extracted by heating silver oxide.
Thus, the extraction method suitable for each metal is decided by how strongly they react with other elements, especially oxygen and carbon.
Q3. Differentiate between roasting and calcination with suitable chemical equations. Provide examples relating to zinc extraction.
Answer:
Roasting:
It is the process of heating an ore in the presence of air or oxygen, mainly used for sulphide ores.
The ore reacts with oxygen to convert the metal sulphide to metal oxide, releasing sulphur dioxide.
Example for zinc:
ZnS (zinc blende) + O₂ → ZnO + SO₂
Calcination:
Involves heating the ore in the absence of air, commonly for carbonate ores.
The ore decomposes to form metal oxide and carbon dioxide.
Example for zinc:
ZnCO₃ (calamine) → ZnO + CO₂
Both roasting and calcination convert ores into oxides, which are then reduced to extract the metal.
Q4. Describe the process and significance of electrolysis in extracting highly reactive metals like aluminium and sodium.
Answer:
Electrolysis is used for metals whose oxides are too stable for reduction by carbon.
The ore is melted to form a liquid that conducts electricity.
Electric current is passed through the molten compound, splitting it into its elements.
At the cathode (negative electrode), positive metal ions gain electrons (reduction) and are deposited as pure metal.
At the anode (positive electrode), non-metal ions lose electrons (oxidation). For example, chlorine gas forms in NaCl electrolysis.
Example:
Sodium extraction from NaCl:
At cathode: Na⁺ + e⁻ → Na
At anode: Cl⁻ – e⁻ → Cl₂↑
Aluminium extraction from Al₂O₃:
At cathode: Al³⁺ + 3e⁻ → Al
At anode: 2O²⁻ → O₂ + 4e⁻
Significance: Enables extraction of metals that otherwise cannot be reduced by conventional chemical means, ensuring their large-scale availability.
Q5. With
reference
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to an activity, explain how moderately reactive metals like copper can be extracted from their oxides using carbon.
Answer:
Moderately reactive metals (Cu, Fe, Zn) are usually extracted by reducing their oxides with carbon (mostly as coke or charcoal).
In the classroom activity for copper:
Copper(II) oxide (CuO), a black solid, is mixed with carbon powder and heated.
The reaction is: CuO + C → Cu + CO
As a result, black CuO turns into reddish-brown copper metal, and carbon monoxide (CO) is released as a gas.
This demonstrates:
Carbon is a reducing agent that removes oxygen from metal oxides.
The reduction technique is cost-effective for metals less reactive than carbon.
The process is analogous for other metals like iron and zinc using their respective oxides.
High Complexity (Analytical & Scenario-Based)
Q6. Imagine you are given three ores: bauxite (Al₂O₃), haematite (Fe₂O₃), and gold dust. Describe how you would extract the pure metal from each, justifying your methods.
Answer:
Bauxite (Al₂O₃):
Aluminium is very reactive and forms a stable oxide.
Extraction is done by electrolysis of molten alumina.
The aluminium ions (Al³⁺) migrate to the cathode and gain electrons to form aluminium metal.
Haematite (Fe₂O₃):
Iron is moderately reactive and can be extracted by chemical reduction.
The ore is mixed with coke and heated in a blast furnace. Carbon acts as the reducing agent:
Fe₂O₃ + 3C → 2Fe + 3CO
Gold dust:
Gold is one of the least reactive metals and is usually found in its pure state.
It can be simply washed and collected (physical separation) or, if combined, easily separated by heating.
Justification:
The chosen method depends on the metal’s reactivity:
Highly reactive → Electrolysis
Moderately reactive → Chemical reduction
Least reactive → Native extraction or simple heating
Q7. A student claims that aluminium can be obtained by reducing alumina (Al₂O₃) with carbon like iron. Critically analyze why this process will not work.
Answer:
Aluminium's position: Aluminium is more reactive than carbon (higher up in the reactivity series).
Stability of alumina: Alumina (Al₂O₃) is an exceptionally stable compound due to the strong aluminium-oxygen bond.
Carbon reduction limitation: Carbon cannot reduce aluminium oxide because it lacks the required reactivity to displace aluminium from its oxide.
Industrial observation: Attempts to heat alumina with carbon result in no reaction.
Practical extraction: Electrolysis of molten alumina is performed as it provides enough energy to break the Al–O bond.
Conclusion: Only highly energetic methods like electrolysis can extract aluminium; carbon reduction is not suitable.
Q8. If you observe a gas that turns limewater milky during the extraction of zinc from its carbonate ore, explain the chemical reactions and processes occurring in detail.
Answer:
The process is calcination of zinc carbonate (ZnCO₃).
Roasting/Calcination Step:
ZnCO₃ (s) (calamine) is heated in the absence of air.
The chemical reaction:
ZnCO₃ (s) → ZnO (s) + CO₂ (g)
Carbon dioxide (CO₂) gas is released during this process.
Limewater Test:
When passed through limewater (Ca(OH)₂), CO₂ forms calcium carbonate (CaCO₃), turning limewater milky:
Ca(OH)₂ (aq) + CO₂ (g) → CaCO₃ (s) + H₂O (l)
Significance:
The observed change confirms the production of CO₂, which indicates calcination is taking place, converting carbonate ore into oxide and preparing it for further reduction.
Q9. Analyse and compare why mercury and gold are found in nature mostly in pure form, while metals like iron are always found as compounds.
Answer:
Reactivity Series:
Gold and mercury are at the bottom of the reactivity series, meaning they are least reactive.
Chemical stability:
These metals do not easily combine with elements like oxygen, sulphur, or carbon dioxide naturally present in the environment.
Formation of compounds:
Metals like iron are more reactive and readily form oxides, sulphides, or carbonates over geological time scales.
Occurrence:
Gold and mercury are found as native elements or as compounds that easily decompose to the metal by simple heating.
Iron, due to its reactivity, is never found in free state but as compounds (e.g., haematite, magnetite).
Conclusion:
The natural occurrence of a metal in pure or compound form is a direct consequence of its chemical reactivity with air, water, and environmental gases over millions of years.
Q10. Given the environmental concerns, evaluate the impact of traditional extraction methods of metals (using coke and electricity) on the environment and suggest greener alternatives.
Answer:
Traditional methods:
Reduction using coke (carbon) releases large amounts of carbon dioxide (CO₂), a greenhouse gas, contributing to global warming.
Electrolysis uses substantial electric energy, which is often derived from fossil fuels, indirectly increasing carbon emissions.
Other impacts:
Mining and extraction generate waste (slag, gangue), cause land degradation, and pollute air and water sources.
Release of sulphur dioxide (SO₂) during roasting leads to acid rain.
Greener alternatives:
Utilizing renewable energy sources (solar, wind) for electrolysis reduces the carbon footprint.
Bioleaching and phytomining: Employ bacteria or plants to extract metals, minimizing energy use and pollution.
Recycling and reusing metals drastically reduces environmental impact.
