From rocks to rocket parts — how metals are pulled out of the Earth. With an interactive Ellingham ΔG calculator, froth flotation diagrams, thermodynamic reasoning, and 20+ real-life examples.
Before diving in, here is the whole chapter on one page. Every metallurgical process is one of five steps: concentrate, convert to oxide, reduce, refine, use.
Metallurgy = extracting a metal from its ore. Steps: (1) concentration removes gangue, (2) calcination or roasting converts the ore to oxide, (3) reduction converts oxide to metal — chosen using the Ellingham diagram, (4) refining purifies the metal, (5) uses and environmental management. Thermodynamics (ΔG = ΔH − TΔS) tells us which reducing agent works at which temperature.
Mineral: A naturally occurring chemical substance (usually an inorganic compound) found in the Earth's crust. Example: clay, quartz, mica.
Ore: A mineral from which a metal can be extracted profitably. All ores are minerals, but not all minerals are ores. Example: bauxite (Al₂O₃·2H₂O) is an ore of aluminium; clay is also an aluminium mineral but not an ore.
Gangue: The unwanted impurities (sand, clay, rock) present in an ore.
Matrix: The rocky material in which the ore is embedded.
Metallurgy: The entire science and technology of extracting a metal from its ore and refining it.
Most metals are reactive (except Au, Ag, Pt — noble metals) and occur in the Earth's crust as compounds, not as free elements.
| Metal | Ore | Chemical Formula |
|---|---|---|
| Aluminium | Bauxite | Al₂O₃·2H₂O |
| Aluminium | Cryolite | Na₃AlF₆ |
| Iron | Haematite | Fe₂O₃ |
| Iron | Magnetite | Fe₃O₄ |
| Iron | Siderite | FeCO₃ |
| Copper | Copper pyrites (chalcopyrite) | CuFeS₂ |
| Copper | Copper glance | Cu₂S |
| Zinc | Zinc blende | ZnS |
| Zinc | Calamine | ZnCO₃ |
| Lead | Galena | PbS |
| Silver | Argentite | Ag₂S |
| Mercury | Cinnabar | HgS |
| Magnesium | Dolomite | CaCO₃·MgCO₃ |
| Titanium | Ilmenite | FeTiO₃ |
Minerals are natural compounds; ores are profitable minerals. Gangue = unwanted impurity. Reactive metals occur as oxides/sulphides/carbonates; noble metals occur native. Learn the formula of bauxite, haematite, zinc blende, galena and cinnabar.
Concentration: The process of removing gangue (unwanted impurities) from the powdered ore, based on differences in physical or chemical properties between the ore and the gangue.
Principle: Difference in specific gravity (density) of the ore and the gangue.
Method: Powdered ore is washed with a stream of water. The lighter gangue particles are carried away, while the heavier ore particles settle down.
Used for: Oxide ores (haematite Fe₂O₃, cassiterite SnO₂, gold).
Limitation: Only works when there is a clear density difference between ore and gangue.
Principle: Difference in magnetic properties.
Method: Powdered ore is carried on a belt past a magnetic roller. Magnetic particles (the ore) stick to the roller and fall close by; non-magnetic gangue falls further away.
Used for: Magnetic ores such as magnetite (Fe₃O₄), pyrolusite (MnO₂), and for removing magnetic impurities such as iron tungstate from cassiterite.
Principle: Difference in wetting ability — sulphide ore particles are wetted by oil (hydrophobic), gangue particles are wetted by water (hydrophilic).
Method: Powdered ore + water + collectors (pine oil, xanthates, fatty acids) + frothers are agitated in a tank. Air is blown in, forming froth. The oil-coated ore particles rise with the froth and are skimmed off; the gangue settles at the bottom.
Used for: Sulphide ores — zinc blende (ZnS), galena (PbS), copper pyrites (CuFeS₂), cinnabar (HgS).
Depressants and activators: A depressant (e.g., NaCN) prevents a particular mineral from floating, enabling separation of two sulphide ores. An activator (e.g., CuSO₄) makes a mineral float.
Principle: The ore (or the gangue) is dissolved selectively in a suitable reagent, then the metal is recovered from solution.
Examples:
Advantage: Leaching can be applied to low-grade ores where other methods are uneconomical.
Concentration methods: hydraulic washing (density), magnetic separation (magnetism), froth flotation (wetting — for sulphide ores), leaching (chemical dissolution — for low-grade ores). Froth flotation is the most important industrial method for sulphide ores.
Most reduction methods work best on metal oxides. Carbonates, sulphides and hydrated oxides are therefore first converted to oxides by heating.
Calcination: Heating the ore strongly in the absence or limited supply of air, to remove volatile matter (water, CO₂, etc.) and to convert carbonates and hydrated oxides into oxides.
Examples:
Used for: Carbonate ores and hydrated oxide ores.
Roasting: Heating the ore strongly in the presence of excess air (oxygen), to convert sulphide ores into oxides and to remove volatile impurities such as arsenic and sulphur as their volatile oxides.
Examples:
Used for: Sulphide ores.
By-product: SO₂ is used to manufacture H₂SO₄ (Contact process) — an important economic and environmental point.
| Feature | Calcination | Roasting |
|---|---|---|
| Air supply | Absent or limited | Excess air / oxygen |
| Applied to | Carbonate and hydrated oxide ores | Sulphide ores |
| Main change | Decomposition; removal of H₂O and CO₂ | Oxidation of sulphide to oxide |
| Example | ZnCO₃ → ZnO + CO₂ | 2ZnS + 3O₂ → 2ZnO + 2SO₂ |
| Products | Metal oxide + volatile gas | Metal oxide + SO₂ |
Do not confuse the two: calcination = no air (think "calcination = closed oven"), roasting = excess air (think "roasting = open flame"). A sulphide ore is always roasted, never calcined. A carbonate ore is always calcined.
Calcination = heating without/limited air, for carbonates and hydrated oxides (removes CO₂ and H₂O). Roasting = heating in excess air, for sulphides (forms oxide + SO₂). Both produce a metal oxide ready for reduction.
ΔG° = ΔH° − TΔS°
A reaction is spontaneous when ΔG° is negative. For the formation of a metal oxide:
Metal + O₂ → Metal oxide (ΔG° becomes more negative as the oxide becomes more stable)
The more negative the ΔG° of formation, the more stable the oxide, and the harder it is to reduce that metal.
Ellingham diagram: A plot of ΔG° of formation of oxides (per mole of O₂ consumed) against temperature, for various metals.
Features:
A metal can reduce the oxide of another metal if its own oxide line lies BELOW the other oxide line at that temperature.
Important consequences:
Choose a reaction (or enter your own ΔH° and ΔS°), set the temperature, and see ΔG°. Also shows the temperature at which ΔG° = 0.
ΔG° = ΔH° − TΔS°. A lower Ellingham line means a more stable oxide. The metal whose line is lower can reduce the oxide whose line is above it. The CO line slopes downward because entropy increases; it crosses many metal oxide lines, making carbon a versatile reducing agent — but not for Al₂O₃ or MgO.
Method: The oxide ore is heated with coke (carbon) — usually in a blast furnace.
Limitation: Carbon cannot reduce oxides of very reactive metals such as Al, Mg, Na, Ca — their Ellingham lines lie far below the carbon line.
In the blast furnace, CO is the actual reducing agent at lower temperatures:
CO is a better reducing agent than carbon at moderate temperatures because its ΔG° line lies below the metal oxide lines there.
Method: A metal with a more negative ΔG° of oxide formation (usually Al) reduces the oxide of another metal. Strongly exothermic — the metal comes out molten.
Use: Welding railway tracks, producing Cr and Mn (metals that carbon cannot reduce cleanly, since carbon forms carbides).
Method: For very reactive metals (Na, K, Ca, Mg, Al), the oxide or chloride is electrolysed in the molten state.
Note: Electrolysis of aqueous solutions does not work for these metals — water would be reduced first (hydrogen is evolved).
Method: The metal is leached into solution as a salt, then reduced to the metal by a more electropositive metal or by electrolysis.
Advantage: Works on low-grade ores and gives very pure metal.
Some ores reduce themselves without an external reducing agent because part of the ore is already in a lower oxidation state.
| Metal | Preferred reduction method | Reason |
|---|---|---|
| Na, K, Ca, Mg, Al | Electrolytic reduction | Too reactive; carbon cannot reduce them |
| Zn, Fe, Pb, Cu | Reduction with C / CO | Moderately reactive; CO line crosses below them at workable T |
| Cr, Mn | Aluminothermy (Al) | Carbon forms carbides; Al gives a cleaner metal |
| Au, Ag | Hydrometallurgy (cyanide + Zn) | Noble metals; readily form soluble complexes |
| Hg, Cu (from sulphide) | Auto-reduction | Part of the ore is already in a reducible state |
Six reduction routes: carbon (smelting), CO, aluminothermy, electrolytic reduction, hydrometallurgy and auto-reduction. The choice follows the Ellingham diagram: reactive metals need electrolysis, moderately reactive metals use C/CO, and Al is used where carbides would form.
Principle: Difference in boiling points.
Used for: Low-boiling metals such as zinc and mercury. The metal is distilled off, leaving impurities behind. Also used for vacuum distillation of zinc (173 K below its boiling point, under reduced pressure).
Principle: Difference in melting points.
Method: The impure metal is heated on a sloping hearth; the metal melts and flows away, while higher-melting impurities are left behind.
Used for: Easily fusible metals such as tin and lead.
Principle: Selective discharge of ions.
Method: Impure metal is made the anode; a thin strip of pure metal is the cathode; a salt solution of the metal is the electrolyte.
Used for: Cu, Zn, Ag, Au, Al, Ni, Pb. Gives very high purity (99.9%+).
Anode mud: A valuable by-product — contains silver, gold and platinum, recovered commercially.
Principle: Impurities are more soluble in the molten state than in the solid state.
Method: A molten zone is moved slowly along a rod of impure metal. As the zone travels, the metal crystallises behind it in a purer form, while impurities are carried forward in the melt. Repeating many times gives ultra-pure metal; the impure end is cut off.
Used for: Semiconductors — silicon, germanium and gallium, which must be extremely pure for electronics.
Principle: The metal is converted into a volatile compound, which is then decomposed to give pure metal.
Mond process (nickel):
van Arkel process (Ti, Zr):
Principle: Different components move at different rates through a stationary phase.
Used for: Purification of small quantities of metals and separation of mixtures — for example, column chromatography and gas chromatography in analytical metallurgy.
| Refining method | Principle | Best used for |
|---|---|---|
| Distillation | Boiling point difference | Zn, Hg (low boiling) |
| Liquation | Melting point difference | Sn, Pb (low melting) |
| Electrolytic | Selective discharge | Cu, Zn, Ag, Au, Ni |
| Zone refining | Solubility of impurity in melt vs solid | Si, Ge, Ga |
| Vapour phase (Mond) | Volatile carbonyl | Ni |
| Vapour phase (van Arkel) | Volatile iodide | Ti, Zr |
Refining: distillation (boiling point), liquation (melting point), electrolytic (selective discharge, highest purity for metals), zone refining (for semiconductors), vapour-phase Mond (Ni) and van Arkel (Ti, Zr), plus chromatography. Impure metal is always the anode in electrolytic refining.
Aluminium — light, corrosion-resistant, used in transport and power cables. Copper — best affordable conductor, wiring and alloys (brass, bronze). Zinc — galvanising and dry cells. Iron — structural backbone of modern industry; steel is its most important form.
1. Sulphur dioxide: Roasting of sulphide ores releases SO₂, a major cause of acid rain.
2. Carbon dioxide and the greenhouse effect: Smelting with carbon and coke produces large amounts of CO₂.
3. Particulate matter: Dust and fumes from crushing, grinding and smelting.
4. Heavy-metal contamination: Arsenic, lead, cadmium and mercury in mine tailings pollute soil and groundwater.
5. Energy consumption: Electrolytic processes (Al, Na) consume enormous electricity.
6. Land degradation: Open-cast mining destroys landscapes and habitats.
Recycling one aluminium can saves enough energy to run a television for about three hours. Aluminium can be recycled indefinitely without losing quality — which is why it is one of the most recycled materials on Earth.
Metallurgy causes SO₂ emissions (acid rain), CO₂ emissions (greenhouse effect), particulate pollution, heavy-metal contamination and energy demand. Solutions: convert SO₂ to H₂SO₄, use precipitators and scrubbers, prefer leaching and recycling, and treat tailings properly.
"C-CO-R-R" — Concentrate → Convert to Oxide → Reduce → Refine.
Read it as "see-co-are-are".
"Calcination = Closed oven" (no air, carbonates)
"Roasting = Roaring flame" (excess air, sulphides)
"Froth floats the SULPHIDE" — sulphide ore is hydrophobic (oil-loving) and rises with the froth; gangue is hydrophilic and sinks.
"LOWER line WINS" — the metal whose oxide line is lower has more affinity for oxygen and can reduce the oxide above it.
"Aluminium sits at the bottom" — hence the thermite process.
"CO goes DOWN" — 2C + O₂ → 2CO increases the number of gas moles, so entropy increases and the slope is negative.
"D-L-E-Z-V-C" — Distillation, Liquation, Electrolytic, Zone, Vapour phase, Chromatography.
"Mond = CO + Nickel" (Mond process: Ni(CO)₄)
"van Arkel = Iodine + Titanium" (TiI₄)
"IMPURE = ANODE" — the impure metal dissolves; pure metal deposits on the cathode.
Use "Calcination = Closed oven" and "Roasting = Roaring flame", "LOWER line WINS" for Ellingham, "CO goes DOWN" for the negative slope, and "IMPURE = ANODE" for electrolytic refining.
Q1. Why is froth flotation used for sulphide ores and not for oxide ores?
Froth flotation depends on the difference in wetting ability. Sulphide ore particles are preferentially wetted by oil (hydrophobic) and are carried up by the froth, whereas the gangue is wetted by water (hydrophilic) and settles.
Oxide ores are hydrophilic (wetted by water), so they do not attach to oil-coated air bubbles and do not float. Hence froth flotation is not effective for oxide ores — hydraulic washing or magnetic separation is used instead.
Q2. Distinguish between calcination and roasting. Give one equation for each.
Heating the ore strongly in the absence or limited supply of air to remove volatile matter (H₂O, CO₂). Applied to carbonate and hydrated oxide ores.
ZnCO₃ → ZnO + CO₂
Heating the ore strongly in the presence of excess air to convert sulphides to oxides and to remove volatile impurities. Applied to sulphide ores.
2ZnS + 3O₂ → 2ZnO + 2SO₂
(i) Air supply: absent/limited vs excess. (ii) Ore type: carbonate/hydrated vs sulphide. (iii) Main change: decomposition vs oxidation. (iv) Gas released: CO₂/H₂O vs SO₂.
Q3. Explain why carbon can reduce zinc oxide but not aluminium oxide.
According to the Ellingham diagram, a reducing agent can reduce a metal oxide only if the ΔG° line of the reducing agent's oxide lies below the metal oxide line at that temperature.
The line for 2C + O₂ → 2CO slopes downward. It crosses below the ZnO line at about 1100 K. Above this temperature, ΔG° for the reduction ZnO + C → Zn + CO becomes negative, so carbon can reduce ZnO.
The Al₂O₃ line lies far below the CO line at all practical temperatures. The reduction 2Al₂O₃ + 3C → 4Al + 3CO₂ therefore has a large positive ΔG° — it is not feasible. Aluminium must be obtained by electrolytic reduction of Al₂O₃ in molten cryolite instead.
Q4. Describe the various methods used for concentrating ores, with examples.
Based on the difference in specific gravity. A stream of water carries away the lighter gangue, while the heavier ore settles. Used for oxide ores such as haematite and cassiterite.
Based on magnetic properties. Magnetic ore particles stick to a magnetic roller and are separated from non-magnetic gangue. Used for magnetite (Fe₃O₄) and pyrolusite (MnO₂).
Based on wetting ability. Powdered ore is agitated with water, oil and a frother, and air is blown through. Oil-coated sulphide particles rise with the froth and are skimmed off; the hydrophilic gangue settles. Used for ZnS, PbS, CuFeS₂, HgS. Depressants (NaCN) and activators (CuSO₄) allow separation of two sulphide ores.
Chemical concentration by selective dissolution. (i) Bauxite is treated with hot NaOH: amphoteric Al₂O₃ dissolves as sodium aluminate while Fe₂O₃ and TiO₂ remain insoluble. (ii) Gold and silver are leached with dilute NaCN in the presence of air to form soluble cyanide complexes, from which the metal is displaced by zinc. Leaching is suitable for low-grade ores.
Q5. Discuss the different methods of refining metals.
Distillation separates by boiling point — used for zinc and mercury; vacuum distillation is used for zinc. Liquation separates by melting point — the metal melts on a sloping hearth and flows away from higher-melting impurities; used for tin and lead.
Impure metal is the anode, pure metal the cathode, and a salt solution of the metal the electrolyte. The anode dissolves and pure metal deposits on the cathode. More electropositive impurities remain in solution; less electropositive ones (Ag, Au) settle as anode mud. Used for Cu, Zn, Ag, Au, Ni, Al.
Based on the fact that impurities are more soluble in the molten state. A molten zone is moved along a rod of impure metal; pure metal crystallises behind the zone while impurities move forward. Repeated passes give ultra-pure metal. Used for Si, Ge, Ga for semiconductors.
The metal is converted into a volatile compound and then decomposed. Mond process: Ni + 4CO → Ni(CO)₄ at 330–350 K, then Ni(CO)₄ → Ni + 4CO at 450–470 K. van Arkel process: Ti + 2I₂ → TiI₄ at ~870 K, then TiI₄ → Ti + 2I₂ on a hot filament at ~1800 K. Used for Ni, Ti, Zr.
Q6. What is the Ellingham diagram? Explain how it is used to choose a reducing agent.
The Ellingham diagram is a plot of ΔG° of formation of oxides (per mole of O₂ consumed) against temperature, for various metals. Since ΔG° = ΔH° − TΔS°, the plots are approximately straight lines.
For most metal oxides the slope is positive, because a gas is consumed and entropy decreases. For 2C + O₂ → 2CO the slope is negative, because 1 mole of gas gives 2 moles of gas (entropy increases). Kinks appear at melting or boiling points.
A metal can reduce the oxide of another metal if its own oxide line lies below the other oxide line at that temperature. The lower line corresponds to a more negative ΔG° and a more stable oxide, i.e., a stronger affinity for oxygen. Where two lines cross, reduction becomes feasible just above that temperature.
(i) Carbon (as CO) reduces ZnO, FeO, PbO and Cu₂O above the crossing temperature, which is why coke is used in the blast furnace. (ii) Aluminium, whose line is very low, reduces Fe₂O₃, Cr₂O₃ and Mn₃O₄ — the thermite process. (iii) Carbon cannot reduce Al₂O₃ or MgO at any practical temperature, so these metals are obtained by electrolytic reduction.
Selected exercise questions from the LEGACY NCERT Chapter 6 (General Principles and Processes of Isolation of Elements). Removed from the current syllabus but useful for competitive exams.
Mineral: A naturally occurring chemical substance found in the Earth's crust, usually inorganic. Examples: clay, quartz, mica, bauxite.
Ore: A mineral from which a metal can be extracted profitably and economically. Examples: bauxite (Al₂O₃·2H₂O), haematite (Fe₂O₃), zinc blende (ZnS).
Key difference: All ores are minerals, but not all minerals are ores. Clay contains aluminium but extracting aluminium from clay is not profitable, so clay is a mineral, not an ore.
Froth flotation works on the difference in wetting ability of the particles.
Sulphide ore particles are preferentially wetted by oil (they are hydrophobic), so when the mixture is agitated with water, oil and a frother and air is blown in, the oil-coated ore particles attach to air bubbles and rise with the froth.
Gangue particles (sand, clay, rock) are hydrophilic — they are wetted by water and remain in the water, settling at the bottom.
Thus the ore is selectively floated and separated. This method is cheap, efficient and works well on finely powdered sulphide ores, which is why it is the standard industrial method for ZnS, PbS, CuFeS₂ and HgS.
(a) Roasting of zinc blende: 2ZnS + 3O₂ → 2ZnO + 2SO₂
(b) Calcination of limestone: CaCO₃ → CaO + CO₂
(c) Thermite reduction of haematite: Fe₂O₃ + 2Al → 2Fe + Al₂O₃ + heat
In (a) excess air is used to oxidise the sulphide to oxide. In (b) the ore is heated in the absence of air to drive off CO₂. In (c) aluminium, whose oxide has a more negative ΔG° of formation, reduces iron oxide; the reaction is highly exothermic and the iron is obtained molten.
Chromatography: A technique for separating the components of a mixture based on their different rates of movement through a stationary phase while carried by a mobile phase. Components with a greater affinity for the stationary phase move more slowly.
Application in metallurgy: Column chromatography and gas chromatography are used to purify small quantities of metals and to separate and identify metal ions in mixtures. It is especially useful in analytical metallurgy, where very small samples must be analysed — for example, separating and estimating the metal ions present in an alloy or a mineral sample.
(a) Depressant: A substance that prevents a particular mineral from being floated, so that two sulphide minerals present together can be separated. Example: sodium cyanide (NaCN) is used as a depressant to prevent zinc sulphide from floating while lead sulphide floats, allowing separation of galena from zinc blende.
(b) Collector: A substance that selectively adsorbs on the surface of the ore particles and makes them hydrophobic (oil-loving), so they attach to air bubbles. Examples: xanthates, fatty acids, pine oil.
(c) Frother: A substance that stabilises the froth so that the ore particles can be carried to the top and skimmed off. Examples: pine oil, cresylic acid, alcohols.
(a) Gangue: The unwanted impurities (sand, clay, rock) present in an ore, which must be removed during concentration. Examples: SiO₂, Al₂O₃, CaCO₃ mixed with the ore.
(b) Slag: The fusible waste product formed when flux reacts with the gangue during smelting. It is lighter than the molten metal, floats on top and is removed. Example: CaSiO₃ formed from CaO and SiO₂ in the blast furnace.
(c) Flux: A substance added to the ore during smelting to combine with the gangue and form a fusible slag. A basic flux (CaO, MgO) removes acidic gangue (SiO₂); an acidic flux (SiO₂) removes basic gangue (CaO, FeO).
The Ellingham diagram plots ΔG° of formation of oxides per mole of O₂ against temperature. Its roles are:
(1) Comparing oxide stability: A lower line means a more negative ΔG° of formation, i.e., a more stable oxide and a greater affinity of the metal for oxygen.
(2) Selecting a reducing agent: A metal can reduce the oxide of another metal only if its own oxide line lies below the other's line at that temperature. This is how the choice between carbon, CO and aluminium is made.
(3) Choosing the temperature: Where the reducing agent's line crosses the metal oxide line, reduction becomes feasible just above that temperature — this sets the furnace temperature.
(4) Explaining limitations: It shows clearly why carbon cannot reduce Al₂O₃ or MgO, and why electrolysis is needed for very reactive metals.
The Mond process is a vapour-phase refining method based on the formation and decomposition of a volatile compound.
Step 1 — Formation of nickel tetracarbonyl:
Impure nickel is heated in a stream of carbon monoxide at
330–350 K.
Ni + 4CO → Ni(CO)₄ (volatile)
Step 2 — Decomposition: The volatile Ni(CO)₄
is passed over a surface heated to 450–470 K, where it
decomposes to give pure nickel.
Ni(CO)₄ → Ni + 4CO
The carbon monoxide is recycled. Impurities do not form volatile carbonyls, so they are left behind. This gives nickel of very high purity.
According to the Ellingham diagram, carbon reduces zinc oxide only above the temperature at which the line for 2C + O₂ → 2CO crosses below the line for 2Zn + O₂ → 2ZnO.
The CO line slopes downward (entropy increases), while the ZnO line slopes upward (entropy decreases). Below the crossing temperature (roughly 1100 K), the ZnO line lies below the CO line, so ΔG° for the reduction ZnO + C → Zn + CO is positive — the reaction is not spontaneous.
Above the crossing temperature, ΔG° becomes negative and reduction proceeds. Therefore zinc oxide must be reduced with coke at a high temperature (about 1100–1500 K), not at low temperature.
(a) Zone refining for Si and Ge: These metals are used in semiconductors and must be extremely pure (impurities in parts per billion). Zone refining relies on impurities being more soluble in the molten state than in the solid state. By moving a molten zone repeatedly along the rod, impurities are swept to one end, giving ultra-high purity.
(b) Impure metal as anode: The impure metal dissolves into the electrolyte as ions (M → Mⁿ⁺ + ne⁻) and pure metal deposits on the cathode (Mⁿ⁺ + ne⁻ → M). Making the impure metal the anode means the metal is continuously replenished in solution and purified at the cathode. More electropositive impurities stay in solution; less electropositive ones settle as anode mud.
(c) Aluminium in the thermite process: Aluminium has a very negative ΔG° of formation of Al₂O₃, so its Ellingham line lies well below those of FeO, Cr₂O₃ and Mn₃O₄. Aluminium can therefore reduce these oxides. The reaction is highly exothermic, producing molten metal — useful for welding railway tracks in remote locations.
| Froth flotation | Sulphide ores | Based on wetting ability |
| Magnetic separation | Fe₃O₄, MnO₂ | Based on magnetism |
| Hydraulic washing | Oxide ores | Based on specific gravity |
| Leaching | Bauxite, Au, Ag | Selective dissolution |
| CO line slope | Negative | Entropy increases (1 gas → 2 gases) |
| Metal oxide line slope | Positive | Entropy decreases (gas consumed) |
| Mond process temperature | 330–350 K / 450–470 K | Formation / decomposition of Ni(CO)₄ |
| van Arkel temperature | ~870 K / ~1800 K | Formation / decomposition of TiI₄ |
| ΔG° | Standard Gibbs energy change (kJ/mol) |
| ΔH° | Standard enthalpy change (kJ/mol) |
| ΔS° | Standard entropy change (J/K/mol) |
| T | Temperature (K) |
| Ore | Profitable mineral |
| Gangue | Unwanted impurity in ore |
| Flux | Added to remove gangue as slag |
| Slag | Fusible waste from flux + gangue |
| Anode mud | Impurity sludge in electrolytic refining |
(A) Both true, R explains A. (B) Both true, R does NOT explain A. (C) A true, R false. (D) A false, R true.
All key equations and facts in one place — perfect for last-minute revision before competitive exams.