General Principles and Processes of Isolation of Elements — NEET Chemistry MCQs with answers

General Principles and Processes of Isolation of Elements is a Chemistry chapter in the NEET (UG) syllabus. NEET720 has 197 reviewed practice questions on it, each with a quick answer and a step-by-step explanation. The 8 questions below are free and fixed, so you can bookmark this page; the full chapter, plus mistake tracking and spaced revision, is in the app.

Topics covered

Ore Concentration · Extraction Principles · Pyrometallurgy · Electrometallurgy · Hydrometallurgy · Refining · Occurrence of Metals · Concentration of Ores · Calcination and Roasting · Thermodynamics of Reduction · Extraction of Iron · Extraction of Copper · Extraction of Aluminium · Refining · Extraction of Zinc · Extraction of Metals · Qualitative Analysis · Concentration of ore · Extraction of crude metal · Thermodynamics of metallurgy · Refining of metals · Thermodynamics of Metallurgy · Extraction Processes

8 free General Principles and Processes of Isolation of Elements practice questions with answers

Choose an answer in your head before opening it. Each explanation says why the correct option is right and, where relevant, why the tempting wrong option is wrong.

1. Question 1 · medium · Ore Concentration

   Froth flotation is used to concentrate sulfide ores (like galena, PbS, or copper pyrites, CuFeS2) by exploiting differences in surface wettability between the sulfide mineral and the unwanted gangue (rock) material. The process works by:

   • A.Adding a collector (like a xanthate) that selectively coats the sulfide mineral particles, making them water-repellent (hydrophobic) so they attach to air bubbles and rise to the surface froth, while the hydrophilic gangue particles remain wetted by water and sink
   • B.Froth flotation works by simply melting the ore and separating components by density, similar to smelting
   • C.The gangue material is made hydrophobic and floats, while the valuable sulfide mineral is made hydrophilic and sinks
   • D.Froth flotation requires no chemical reagents at all, relying purely on mechanical agitation
   Show answer and explanation

   Answer: A. Adding a collector (like a xanthate) that selectively coats the sulfide mineral particles, making them water-repellent (hydrophobic) so they attach to air bubbles and rise to the surface froth, while the hydrophilic gangue particles remain wetted by water and sink

   A collector reagent (like a xanthate) selectively adsorbs onto the surface of the sulfide mineral particles, making them hydrophobic so they attach to rising air bubbles and are carried to the froth layer at the top, which is skimmed off, while the hydrophilic gangue particles remain wetted by water and settle to the bottom.

   In froth flotation, finely powdered ore is suspended in water, and a small quantity of a collector reagent (commonly a xanthate, such as sodium ethyl xanthate) is added, which selectively adsorbs onto the surface of the sulfide mineral particles (through specific chemical affinity between the collector and the sulfide surface), rendering these particles hydrophobic (water-repellent) while leaving the gangue (unwanted silicate/oxide rock material) particles largely unaffected, remaining hydrophilic (water-wetting). Air is then bubbled through the mixture (often with a frother reagent, like pine oil, added to help stabilize the resulting froth); the hydrophobic sulfide mineral particles preferentially attach to these rising air bubbles and are carried up to the surface, forming a stable, mineral-laden froth layer that can be skimmed off and further processed, while the hydrophilic gangue particles remain wetted by the surrounding water and settle to the bottom of the flotation cell, achieving an effective physical separation and concentration of the valuable sulfide mineral from the unwanted gangue material.

   Common mistake: Reversing which component (mineral vs gangue) becomes hydrophobic/floats versus hydrophilic/sinks

2. Question 2 · easy · Ore Concentration

   Magnetic separation is used to concentrate ores where either the ore mineral or the gangue is magnetic (e.g., separating magnetite, Fe3O4, from non-magnetic gangue, or separating non-magnetic tinstone/cassiterite from magnetic iron-bearing impurities). This method works by:

   • A.Passing the crushed ore over a conveyor belt with a magnetic roller/drum, causing the magnetic component to be attracted and deflected into a separate collection bin from the non-magnetic component
   • B.Magnetic separation works by melting the ore and using magnetism to separate liquid components
   • C.This method requires the ore and gangue to have identical magnetic properties for it to work
   • D.Magnetic separation is used exclusively for non-metallic minerals, never for metal-bearing ores
   Show answer and explanation

   Answer: A. Passing the crushed ore over a conveyor belt with a magnetic roller/drum, causing the magnetic component to be attracted and deflected into a separate collection bin from the non-magnetic component

   Crushed ore is passed over a moving conveyor belt equipped with a magnetic roller; the magnetic component of the ore is attracted and deflected by the magnet into a separate collection zone, while the non-magnetic component continues along its original trajectory, achieving physical separation based on this magnetic property difference.

   Magnetic separation exploits a difference in magnetic susceptibility between the desired ore mineral and the unwanted gangue material (or, in some applications, between the desired ore mineral and an unwanted magnetic impurity mineral, with the roles of 'magnetic' and 'non-magnetic' components varying depending on the specific ore system). The finely crushed ore mixture is fed onto a moving conveyor belt, at the end of which a magnetic roller/drum is positioned. As the ore particles pass over this magnetic roller, the magnetic component of the mixture is attracted toward and deflected by the magnetic field, causing it to fall into a different, separate collection zone compared to the non-magnetic component (which, unaffected by the magnetic field, continues along its original, undeflected trajectory as it leaves the belt), achieving an effective physical separation based purely on this differential magnetic response, without requiring any chemical reagents or melting of the ore material.

   Common mistake: Assuming magnetic separation requires melting the ore or works only for non-metallic minerals

3. Question 3 · hard · Extraction Principles

   An Ellingham diagram plots the standard Gibbs free energy change (ΔG°) for the formation of a metal oxide (per mole of O2 consumed) against temperature. For a given metal oxide MO, carbon can serve as an effective reducing agent at a specific temperature if:

   • A.At that temperature, the ΔG° line for the formation of CO (or CO2) from carbon lies BELOW the ΔG° line for the formation of MO from the metal, indicating carbon's oxidation is more thermodynamically favourable, allowing it to reduce MO to the metal
   • B.Carbon can reduce any metal oxide at any temperature, regardless of the relative positions of the Ellingham diagram lines
   • C.Carbon can only reduce a metal oxide if the carbon line is ABOVE the metal oxide line at the relevant temperature
   • D.Temperature has no effect on which reducing agent is most effective for a given metal oxide
   Show answer and explanation

   Answer: A. At that temperature, the ΔG° line for the formation of CO (or CO2) from carbon lies BELOW the ΔG° line for the formation of MO from the metal, indicating carbon's oxidation is more thermodynamically favourable, allowing it to reduce MO to the metal

   On an Ellingham diagram, a lower (more negative) ΔG° line indicates a more thermodynamically favourable/stable oxide formation; for carbon to successfully reduce a metal oxide MO, the carbon oxidation line must lie below the MO line at the relevant temperature, meaning carbon's affinity for oxygen (forming CO/CO2) is greater than the metal's affinity for oxygen at that specific temperature, making the overall reduction reaction thermodynamically favourable.

   The Ellingham diagram plots ΔG° (standard Gibbs free energy change) for the reaction of various elements with oxygen (forming their respective oxides, normalized per mole of O2 consumed) as a function of temperature. A more negative ΔG° value indicates a thermodynamically more stable (more favourable to form) oxide. For a reducing agent (like carbon) to successfully reduce a target metal oxide MO to the free metal M (with carbon itself being oxidized to CO or CO2), the overall combined reaction must have a net negative ΔG°, which occurs specifically when the ΔG° line for carbon's oxidation (to CO/CO2) lies BELOW the ΔG° line for the metal's oxidation (to MO) at the temperature in question — this indicates that carbon has a greater thermodynamic affinity for oxygen than the metal does at that temperature, making it energetically favourable for carbon to 'steal' the oxygen from the metal oxide, reducing it to the free metal while carbon itself becomes oxidized. Since the carbon-to-CO line on a typical Ellingham diagram has an unusual, notably downward-sloping trajectory (due to the increasing entropy associated with converting solid carbon plus gaseous O2 into gaseous CO, an entropy-favourable process that becomes increasingly thermodynamically favoured as temperature rises, in contrast to most metal oxide formation reactions, which typically show a less negative, often nearly flat or even slightly upward-sloping ΔG° trend with increasing temperature), the carbon line eventually crosses below and remains below many metal oxide lines above a certain characteristic 'crossover temperature,' explaining why carbon (used in blast furnace iron/other metal extraction) becomes an effective, thermodynamically favourable reducing agent specifically above this particular temperature threshold for a given metal oxide, but may be ineffective (or even work in reverse) at lower temperatures.

   Common mistake: Reversing the correct relative line position (above vs below) needed for a reducing agent to successfully reduce a target metal oxide

4. Question 4 · medium · Pyrometallurgy

   In the blast furnace extraction of iron, iron ore (Fe2O3), coke (carbon), and limestone (CaCO3) are charged into the top of the furnace, while hot air is blown in near the bottom. The actual reducing agent for iron oxide in the upper, cooler regions of the furnace is predominantly:

   • A.Carbon monoxide (CO), formed from the reaction of coke with oxygen/CO2 lower in the furnace, which then rises and reduces Fe2O3 to iron in the cooler upper zones via indirect reduction reactions
   • B.Solid carbon (coke) itself is the exclusive reducing agent throughout the entire furnace, with no role for any gaseous intermediate
   • C.Limestone (CaCO3) is the primary reducing agent for iron oxide
   • D.Hot air blown into the furnace directly reduces the iron oxide
   Show answer and explanation

   Answer: A. Carbon monoxide (CO), formed from the reaction of coke with oxygen/CO2 lower in the furnace, which then rises and reduces Fe2O3 to iron in the cooler upper zones via indirect reduction reactions

   In the hot lower zone, coke burns with the blown-in hot air to form CO2, which then reacts with more coke to form CO; this CO gas rises through the furnace and, in the cooler upper zones, indirectly reduces the descending iron ore (Fe2O3) to iron through a series of stepwise reduction reactions, itself being oxidized to CO2 in the process.

   The blast furnace operates as a large, continuous countercurrent reactor, with solid materials (ore, coke, limestone) moving downward while hot gases move upward. Near the bottom (tuyere zone), hot air blown into the furnace reacts with coke: C + O2 → CO2 (highly exothermic, generating the intense heat needed for the overall process), and this CO2 further reacts with additional hot coke: CO2 + C → 2CO (an endothermic reaction, consuming some of the generated heat but producing the key reducing gas). This carbon monoxide gas then rises upward through the furnace, passing through progressively cooler zones where it reacts with and reduces the descending iron ore in a series of stepwise indirect reduction reactions (approximately: 3Fe2O3 + CO → 2Fe3O4 + CO2, then Fe3O4 + CO → 3FeO + CO2, then FeO + CO → Fe + CO2, occurring at successively lower temperatures as the ore descends and gas rises through the furnace's temperature gradient), with CO itself being oxidized to CO2 in each step. While some direct reduction by solid carbon also occurs, particularly in the hottest lower regions of the furnace where temperatures are sufficient, the majority of the overall iron oxide reduction is generally understood to occur via this indirect, gas-phase CO-mediated mechanism in the furnace's cooler upper regions, making CO gas the predominant practical reducing agent for iron ore reduction in a blast furnace, an important and specifically emphasized nuance in blast furnace metallurgy.

   Common mistake: Assuming solid coke is the exclusive reducing agent throughout the entire furnace, without recognizing the significant contribution of the gaseous CO-mediated indirect reduction pathway

5. Question 5 · medium · Pyrometallurgy

   In the blast furnace, limestone (CaCO3) is added specifically to react with silica (SiO2) and other acidic gangue impurities present in the iron ore, forming a molten slag (calcium silicate, CaSiO3) that is less dense than molten iron and can be drawn off separately. This slag formation process is best described as:

   • A.An acid-base reaction where limestone first decomposes to basic CaO, which then reacts with acidic SiO2 gangue to form the more easily removable calcium silicate slag, effectively purifying the molten iron by removing this unwanted impurity
   • B.Limestone reacts directly with iron oxide, not with the silica gangue impurity
   • C.The resulting slag is denser than molten iron, sinking below it
   • D.This process has no connection to acid-base chemistry concepts
   Show answer and explanation

   Answer: A. An acid-base reaction where limestone first decomposes to basic CaO, which then reacts with acidic SiO2 gangue to form the more easily removable calcium silicate slag, effectively purifying the molten iron by removing this unwanted impurity

   Limestone first thermally decomposes (CaCO3 → CaO + CO2) to form calcium oxide, a basic oxide, which then reacts with the acidic silica gangue impurity (CaO + SiO2 → CaSiO3) to form molten calcium silicate slag; being less dense than molten iron, this slag floats on top and can be conveniently drawn off separately, purifying the iron product.

   Limestone (CaCO3) added to the blast furnace charge first undergoes thermal decomposition at the furnace's operating temperature: CaCO3 → CaO + CO2, releasing calcium oxide (a basic metal oxide) and CO2 gas (which itself can participate further in the reduction gas chemistry discussed earlier). This calcium oxide then reacts with the acidic gangue impurity silica (SiO2), naturally present in the iron ore, through a classic acid-base (Lux-Flood type) reaction: CaO + SiO2 → CaSiO3 (calcium silicate), forming a molten slag. This slag, being significantly less dense than the molten iron being produced in the furnace, floats as a distinct layer on top of the molten iron pool that accumulates at the bottom of the furnace, allowing the slag to be conveniently and separately drawn off (tapped) through a higher outlet than the molten iron itself, effectively removing this unwanted silica gangue impurity from the final iron product and purifying it, illustrating a practical industrial application of basic acid-base oxide chemistry principles within the broader pyrometallurgical extraction process.

   Common mistake: Assuming limestone reacts with the iron oxide itself, or confusing the relative densities of slag and molten iron

6. Question 6 · medium · Ore Concentration

   In the Bayer's process, bauxite ore (impure Al2O3) is concentrated by digesting it with hot concentrated NaOH solution, which dissolves the amphoteric Al2O3 as soluble sodium aluminate, while impurities like Fe2O3 remain undissolved (as they are not amphoteric) and are filtered off. Pure Al2O3 is then recovered by:

   • A.Diluting the sodium aluminate solution and seeding it with pure Al(OH)3, causing hydrated alumina to precipitate out (reversing the dissolution), which is then filtered and calcined (heated) to give pure Al2O3
   • B.Simply evaporating the sodium aluminate solution to dryness, since sodium aluminate itself is the desired pure product
   • C.Fe2O3 is also amphoteric and dissolves alongside Al2O3, making this separation impossible
   • D.This process does not involve any acid-base chemistry concepts
   Show answer and explanation

   Answer: A. Diluting the sodium aluminate solution and seeding it with pure Al(OH)3, causing hydrated alumina to precipitate out (reversing the dissolution), which is then filtered and calcined (heated) to give pure Al2O3

   Al2O3, being amphoteric, dissolves in hot concentrated NaOH to form soluble sodium aluminate (NaAlO2), separating it from non-amphoteric Fe2O3 impurity; diluting and seeding this solution with Al(OH)3 crystals reverses the dissolution, precipitating pure hydrated alumina, which is then calcined to give pure Al2O3.

   Bauxite ore (impure Al2O3, containing Fe2O3 and other impurities) is digested with hot, concentrated NaOH solution under pressure. Since Al2O3 is amphoteric, it reacts with the strong base to form soluble sodium aluminate: Al2O3 + 2NaOH → 2NaAlO2 + H2O. Fe2O3, being a predominantly basic oxide (not significantly amphoteric under these conditions), does not dissolve appreciably in NaOH and remains as an insoluble solid residue ('red mud'), which is filtered off, achieving the key separation of alumina from this major impurity. The resulting sodium aluminate filtrate is then diluted with water and 'seeded' with a small quantity of previously prepared pure Al(OH)3 crystals, which promotes and accelerates the reverse reaction (hydrolysis/precipitation): NaAlO2 + 2H2O → Al(OH)3↓ + NaOH, causing pure hydrated aluminium hydroxide to precipitate out of solution (this reversal is favoured by dilution, which shifts the amphoteric equilibrium). This Al(OH)3 precipitate is filtered, washed, and then calcined (strongly heated) to drive off water, yielding pure Al2O3 (alumina), which is subsequently used as the feedstock for the electrolytic Hall-Heroult process (discussed earlier regarding cryolite) to produce metallic aluminium.

   Common mistake: Assuming simple evaporation of the sodium aluminate solution would give pure Al2O3, rather than recognizing the specific dilution/seeding precipitation and subsequent calcination steps required

7. Question 7 · easy · Extraction Principles

   Roasting and calcination are both pre-treatment steps used to convert concentrated ore into a suitable form before final reduction, but they differ in a key respect. Roasting is specifically defined as:

   • A.Heating a sulfide ore strongly in the presence of excess air/oxygen, converting the sulfide to the corresponding oxide (and releasing SO2 gas)
   • B.Heating a carbonate/hydrated oxide ore in the absence of air, driving off volatile matter like CO2 or water without providing oxygen for oxidation
   • C.Roasting and calcination are identical processes with no meaningful distinction between them
   • D.Roasting is used exclusively for carbonate ores, never for sulfide ores
   Show answer and explanation

   Answer: A. Heating a sulfide ore strongly in the presence of excess air/oxygen, converting the sulfide to the corresponding oxide (and releasing SO2 gas)

   Roasting specifically involves heating a sulfide ore in the presence of excess air/oxygen, oxidizing the metal sulfide to the corresponding metal oxide while releasing sulfur dioxide gas, preparing the ore for subsequent reduction to the free metal.

   Roasting is a pyrometallurgical pre-treatment step specifically applied to sulfide ores (such as ZnS, PbS, CuFeS2), involving strong heating of the concentrated ore in the presence of excess air or oxygen. This oxidizes the metal sulfide to the corresponding metal oxide (a form generally more amenable to subsequent reduction), while the sulfur content is released as sulfur dioxide gas: for example, 2ZnS + 3O2 → 2ZnO + 2SO2. This is distinguished from calcination, which is typically applied to carbonate ores (like CaCO3, or hydrated oxide ores like bauxite before Bayer's process concentration) and involves heating strongly in the absence (or limited presence) of air, driving off volatile components (such as CO2 from a carbonate, or water from a hydrated oxide) without necessarily involving oxidation, as discussed earlier regarding limestone decomposition (CaCO3 → CaO + CO2) in the blast furnace context. This roasting/calcination distinction (oxidative treatment of sulfides versus non-oxidative thermal decomposition of carbonates/hydrates) is an important, frequently tested nuance in extractive metallurgy.

   Common mistake: Confusing or reversing the roasting and calcination definitions, or applying them to the wrong ore type

8. Question 8 · medium · Extraction Principles

   The thermite reaction, used for welding railway tracks and other high-temperature metal joining applications, involves the highly exothermic reaction: 2Al + Fe2O3 → Al2O3 + 2Fe. This reaction is thermodynamically favourable and self-sustaining because:

   • A.Aluminium has a much greater thermodynamic affinity for oxygen than iron does (Al2O3 is more stable/has more negative Gibbs free energy of formation than Fe2O3), making it energetically favourable for aluminium to displace/reduce the iron from its oxide
   • B.Iron has a greater affinity for oxygen than aluminium, making this reaction thermodynamically unfavourable
   • C.This reaction absorbs a large amount of heat, making it endothermic rather than exothermic
   • D.The thermite reaction has no relationship to the Ellingham diagram concept discussed earlier
   Show answer and explanation

   Answer: A. Aluminium has a much greater thermodynamic affinity for oxygen than iron does (Al2O3 is more stable/has more negative Gibbs free energy of formation than Fe2O3), making it energetically favourable for aluminium to displace/reduce the iron from its oxide

   Aluminium's oxide (Al2O3) is considerably more thermodynamically stable (more negative Gibbs free energy of formation) than iron's oxide (Fe2O3), meaning aluminium has a much stronger affinity for oxygen than iron does; this makes it highly thermodynamically favourable for aluminium to 'steal' oxygen from Fe2O3, reducing it to metallic iron while aluminium itself is oxidized, releasing a very large quantity of heat in this exothermic displacement reaction.

   This reaction directly illustrates the same Ellingham diagram principle discussed earlier regarding selecting an effective reducing agent for a given metal oxide: on an Ellingham diagram, the ΔG° line for aluminium oxide formation (2Al + 3/2 O2 → Al2O3) lies considerably BELOW (more negative) the corresponding line for iron oxide formation (2Fe + 3/2 O2 → Fe2O3) across a wide range of practically relevant temperatures, indicating that aluminium has a substantially greater thermodynamic affinity for oxygen than iron does. This large difference in thermodynamic stability between the two oxides makes the overall displacement reaction (where aluminium reduces Fe2O3 to metallic iron while itself being oxidized to Al2O3) very strongly thermodynamically favourable, with a large negative overall ΔG° and correspondingly a very large amount of heat released (highly exothermic, ΔH ≈ -850 kJ/mol), generating temperatures exceeding 2000°C, sufficient to melt the resulting molten iron product (and the accompanying Al2O3 slag), making this thermite reaction practically useful for specialized high-temperature welding applications (such as joining railway track sections in the field, where the intense, localized heat generated by this self-sustaining reaction can effectively fuse the metal pieces together) and directly connecting back to and illustrating the general Ellingham diagram reasoning discussed earlier for selecting effective reducing agents in metal extraction contexts.

   Common mistake: Reversing the relative oxygen affinity of aluminium versus iron, or assuming the thermite reaction is endothermic rather than strongly exothermic

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