This MCQ module is based on: Occurrence of Metals, Metallurgy and Corrosion
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Occurrence of Metals, Metallurgy and Corrosion
🎓 Class 10
Science
CBSE
Theory
Ch 3 — Metals and Non-metals
⏱ ~23 min
🌐 Language: [gtranslate]
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3.4 Occurrence of Metals
The Earth's crust is the principal reservoir of metals. A few noble metals such as gold, silver, platinum and (sometimes) copper are found in their free state because they are chemically inert. Most other metals react with oxygen, sulphur, carbonate or chloride of the surrounding crust and exist as compounds.
Mineral & Ore: A naturally occurring substance of definite composition containing a metal is called a mineral. Those minerals from which a metal can be extracted conveniently and profitably are called ores. So every ore is a mineral, but not every mineral is an ore.
| Category | Ore | Formula | Metal |
|---|---|---|---|
| Oxides | Haematite | Fe2O3 | Iron |
| Magnetite | Fe3O4 | Iron | |
| Bauxite | Al2O3·2H2O | Aluminium | |
| Pyrolusite | MnO2 | Manganese | |
| Sulphides | Zinc blende | ZnS | Zinc |
| Galena | PbS | Lead | |
| Cinnabar | HgS | Mercury | |
| Carbonates | Limestone | CaCO3 | Calcium |
| Calamine | ZnCO3 | Zinc | |
| Chloride | Rock salt | NaCl | Sodium |
3.4.1 Steps of Metallurgy
Metallurgy comprises the following general steps:
Step 1: Crushing & Grinding
Large lumps of ore are broken into fine powder in crushers and ball-mills to increase the surface area for later steps.
Step 2: Concentration of Ore (Removing Gangue)
Unwanted earthy and rocky impurities associated with the ore are called gangue.
- Hydraulic (gravity) washing: powdered ore is washed with water — dense ore particles sink, lighter gangue flows away. Used for oxide ores of heavy metals.
- Magnetic separation: a magnetic pulley attracts magnetic ore particles (e.g., Fe3O4) while gangue falls off separately.
- Froth flotation: for sulphide ores. Powdered ore is added to water with pine oil and air blown through. The oily sulphide particles rise with the froth while gangue settles.
- Chemical leaching: the ore is dissolved in a suitable reagent (e.g., hot NaOH for bauxite) that dissolves the metal compound leaving gangue behind.
Step 3: Conversion to Oxide
Metal oxides are the easiest compounds to reduce to metal, so non-oxide ores are first converted:
- Roasting — heating a sulphide ore strongly in the presence of excess air:
\(2\text{ZnS}(s) + 3\text{O}_2(g) \xrightarrow{\Delta} 2\text{ZnO}(s) + 2\text{SO}_2(g)\)
- Calcination — heating a carbonate ore in limited supply of air:
\(\text{ZnCO}_3(s) \xrightarrow{\Delta} \text{ZnO}(s) + \text{CO}_2(g)\)\(\text{CaCO}_3(s) \xrightarrow{\Delta} \text{CaO}(s) + \text{CO}_2(g)\)
Step 4: Reduction to Metal
This step depends on where the metal sits in the reactivity series.
| Reactivity | Method | Examples |
|---|---|---|
| Highly reactive (top): K, Na, Ca, Mg, Al | Electrolytic reduction of molten ore (C cannot pull O from them) | \(2\text{Al}_2\text{O}_3 \xrightarrow{\text{electrolysis}} 4\text{Al} + 3\text{O}_2\) (Hall process) |
| Moderately reactive: Zn, Fe, Pb, Cu | Reduction of oxide with carbon (coke) | \(\text{ZnO} + \text{C} \rightarrow \text{Zn} + \text{CO}\) ; \(\text{Fe}_2\text{O}_3 + 3\text{C} \rightarrow 2\text{Fe} + 3\text{CO}\) |
| Less reactive: Hg, Cu (partially) | Heating the sulphide in air is enough (self-reduction) | \(2\text{HgS}+3\text{O}_2\rightarrow 2\text{HgO}+2\text{SO}_2\) then \(2\text{HgO}\xrightarrow{\Delta}2\text{Hg}+\text{O}_2\) |
Step 5: Refining
Metals obtained above are still impure. For high purity, electrolytic refining is used: impure metal is made the anode, a thin strip of pure metal is the cathode, and the electrolyte is a solution of its salt. Pure metal is deposited on the cathode.
3.4.2 The Thermit Reaction
Aluminium, being highly reactive, reduces iron(III) oxide to molten iron with an enormous release of heat:
\(\text{Fe}_2\text{O}_3(s) + 2\text{Al}(s) \rightarrow 2\text{Fe}(l) + \text{Al}_2\text{O}_3(s) + \text{heat}\)
This thermit reaction generates temperatures near 2500°C. The molten iron produced is used on site to weld broken railway tracks and cracked machine parts.
3.5 Corrosion
Corrosion is the slow attack of air, water and acids on a metal's surface.
| Metal | Corrosion product | Appearance |
|---|---|---|
| Iron | Fe2O3·nH2O (rust) | Reddish-brown, flaky |
| Silver | Ag2S (tarnish) | Black coating |
| Copper | CuCO3·Cu(OH)2 | Green patina (e.g., Statue of Liberty) |
Rusting needs both air and water. Without one of them, rusting cannot occur.
3.5.1 Preventing Corrosion
- Painting, oiling, greasing: a simple physical barrier to oxygen and moisture.
- Galvanising: coating iron with a thin layer of zinc. Even if the coat is scratched, Zn continues to protect Fe sacrificially by corroding first (Zn is more reactive).
- Chromium / tin plating: for bumpers, taps, utensils.
- Anodising of aluminium (thick artificial Al2O3).
- Alloying changes the chemistry altogether — stainless steel does not rust.
Alloys
An alloy is a homogeneous mixture of two or more metals (or of a metal with a non-metal like carbon). Alloying can make metals harder, less corrosive, lower-melting or more useful.
| Alloy | Composition | Use |
|---|---|---|
| Brass | Cu + Zn | Decorative articles, utensils |
| Bronze | Cu + Sn | Statues, medals |
| Solder | Pb + Sn | Welding electrical wires (low MP) |
| Stainless steel | Fe + C + Cr + Ni | Utensils, surgical tools (does not rust) |
| Amalgam | Any metal + Hg | Dental fillings; extraction of Au/Ag |
| 22-carat gold | 22 parts Au + 2 parts (Ag/Cu) | Jewellery (pure 24 K gold is too soft) |
Gold purity: Pure gold is 24 carat. Jewellery gold is usually 22 K = 22/24 ≈ 91.7% Au. The remaining 8.3% copper or silver gives it the hardness needed to hold stones and resist deformation.
Worked Examples
Example 1 — Write balanced equations for (a) roasting of ZnS (b) calcination of ZnCO3.
(a) \(2\text{ZnS} + 3\text{O}_2 \xrightarrow{\Delta} 2\text{ZnO} + 2\text{SO}_2\)
(b) \(\text{ZnCO}_3 \xrightarrow{\Delta} \text{ZnO} + \text{CO}_2\)
Example 2 — Why cannot aluminium be obtained by reduction of Al2O3 with carbon?
Solution: Al is a very reactive metal — its affinity for oxygen is far greater than that of carbon. Hence C cannot pull O away from Al2O3. Instead, Al is obtained by electrolysis of molten Al2O3 (Hall-Heroult process).
Example 3 — Explain why the thermit reaction is used for welding cracked rails.
Solution: The reaction Fe2O3 + 2Al → 2Fe + Al2O3 is highly exothermic — it liberates enough heat to produce molten iron on the spot, which fills the gap between the broken rails and sets into a strong weld. No external power supply is needed.
Example 4 — A metal M occurs as its sulphide MS. Outline its complete extraction.
- Crushing & grinding of the sulphide ore.
- Concentration by froth flotation.
- Roasting: 2MS + 3O2 → 2MO + 2SO2.
- Reduction of the oxide with coke: MO + C → M + CO.
- Electrolytic refining to obtain pure M.
Example 5 — Why does a freshly cut sodium surface quickly lose its shine in air?
Solution: Na reacts rapidly with atmospheric O2 and moisture to form Na2O, NaOH and Na2CO3, coating the surface with a dull layer. This is why Na is stored under kerosene.
Example 6 — Write the equation for rusting of iron and name the conditions.
Solution: In the simplified form: 4Fe + 3O2 + 2nH2O → 2(Fe2O3·nH2O). Both oxygen (air) and water (moisture) are needed; electrolytes (dissolved salts, acid fumes) accelerate the process.
Example 7 — Why is galvanisation preferred to ordinary painting to protect an iron bucket?
Solution: Paint merely provides a surface barrier; once scratched, rusting begins there. In galvanisation, Zn is more reactive than Fe, so even if the Zn layer is scratched, Zn continues to corrode preferentially (sacrificially), sparing the iron. Protection is therefore much longer-lasting.
Example 8 — Why is pure gold alloyed with copper or silver for making ornaments?
Solution: 24-carat pure gold is too soft to retain shape or hold gemstones. Adding 8.3% Cu/Ag (to make 22-carat gold) increases hardness and strength while keeping its appearance.
Activity — Conditions Needed for RustingL3 Apply
Predict: In which of the three tubes — (a) iron nail with dry air only, (b) iron nail with boiled water and oil layer (no air), (c) iron nail with ordinary tap water — will rust form?
- Take three test tubes A, B, C each containing an iron nail.
- A: with anhydrous CaCl2 (dries the air above the nail).
- B: with boiled water (no dissolved air) and a thin oil film on top.
- C: with ordinary tap water (normal air + water).
- Leave for a few days and observe.
Observation: Only nail C rusts. A has no water; B has no air. This proves rusting requires both air (oxygen) and water.
Interactive: Choose the Extraction Method L3 Apply
Pick a metal and the tool will suggest the extraction method based on its position in the reactivity series.
Competency-Based Questions
A railway engineer needs to weld a cracked track on a remote hillside where there is no electricity supply.
Q1. L2 Understand Which reaction is used and why is it especially suited here? (2 marks)
The thermit reaction: Fe2O3 + 2Al → 2Fe + Al2O3 + heat. It is highly exothermic, producing molten iron without needing external electricity — ideal for remote sites.
Q2. L3 Apply Balance: Fe2O3 + Al → Fe + Al2O3. (1 mark)
Fe2O3 + 2Al → 2Fe + Al2O3.
Q3. L4 Analyse Explain why sodium cannot be extracted by reducing Na2O with carbon. (2 marks)
Na is very high in the reactivity series — it has a greater affinity for oxygen than carbon does. C cannot pull O away from Na2O. Highly reactive metals like Na, K, Ca, Mg, Al are obtained only by electrolytic reduction of their molten compounds.
Q4. L1 Remember Name one ore each of iron, aluminium, zinc, and mercury. (2 marks)
Iron — Haematite (Fe2O3); Aluminium — Bauxite (Al2O3·2H2O); Zinc — Zinc blende (ZnS) or Calamine (ZnCO3); Mercury — Cinnabar (HgS).
Q5. L5 Evaluate A student claims that painting an iron pipe is better than galvanising because paint is cheaper. Critique. (3 marks)
Paint is cheaper initially, but once the paint is scratched, moisture and air reach the exposed iron directly and rusting starts immediately. Galvanising coats iron with Zn, which is more reactive; even after scratches, the Zn continues to corrode sacrificially and saves the iron for years. Over the lifetime of the pipe, galvanisation is cheaper and far more reliable than paint. The student's reasoning overlooks durability.
Assertion-Reason Questions
Assertion (A): Aluminium is extracted by electrolysis of molten bauxite.
Reason (R): Al is too reactive to be reduced by carbon.
A.
Assertion (A): Galvanised iron does not rust even after the Zn coating is scratched.
Reason (R): Zn is less reactive than Fe.
C. A is true; R is false — Zn is more reactive than Fe, which is why it corrodes sacrificially.
Assertion (A): Roasting is done for sulphide ores.
Reason (R): Sulphide ores are heated in excess air to convert them to oxides.
A.
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