Metals And Non Metals-V

Gangue (also called matrix) refers to the unwanted earthy and rocky impurities present in an ore. These impurities primarily consist of materials like soil, sand (silica, SiO₂), clay, limestone, and other rocky fragments. When an ore is mined, it contains both the desired metal-bearing mineral and gangue. The process of removing gangue from the ore is called concentration, beneficiation, or dressing of the ore. Gangue does not consist of oxides only, metals, or acids.
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Brass is a well-known alloy made primarily of copper (Cu) and zinc (Zn). The proportion of zinc can vary from 5% to 45% depending on the desired properties. Brass is harder and more ductile than pure copper, has excellent corrosion resistance, and has a bright gold-like appearance. It is widely used for making musical instruments, decorative items, locks, hinges, and plumbing fittings. Copper and tin form bronze, while iron and carbon form steel. Lead and tin form solder.
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The metal obtained directly after the extraction process (reduction of the ore) is called crude or blister metal. This metal is not pure; it contains various impurities such as unreacted ore, gangue particles, other metals present in the original ore, and by-products of the reduction process. For example, the iron obtained from a blast furnace (pig iron) contains carbon, silicon, phosphorus, and sulfur as impurities. Therefore, the extracted metal must undergo further purification through refining processes to obtain pure metal.
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Galvanisation is a method of preventing rust by coating iron or steel with a thin layer of zinc. Zinc is more reactive than iron, so it acts as a sacrificial anode. Even if the zinc coating is scratched, the zinc corrodes preferentially, protecting the iron underneath. Other common rust prevention methods include painting, oiling/greasing, chrome plating, alloying (making stainless steel), and using barriers like plastic coatings. Crushing, heating, or dissolving would not prevent rusting.
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In the reactivity series, metals are classified into three broad categories: highly reactive (K, Na, Ca, Mg, Al), moderately reactive (Zn, Fe, Sn, Pb), and least reactive (Cu, Ag, Au, Pt). Iron and zinc fall in the middle of the series. They are not as violently reactive as alkali or alkaline earth metals, but they do react with oxygen, water (steam), and acids. They are never found in the free state in nature and occur as oxides, sulphides, or carbonates.
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Moderately reactive metals (like zinc, iron, lead, and copper) are not found in the free state in nature because they react with atmospheric oxygen, moisture, and other elements over geological time scales. They commonly occur as sulphide ores (e.g., ZnS – zinc blende, PbS – galena) or carbonate ores (e.g., ZnCO₃ – calamine, CuCO₃·Cu(OH)₂ – malachite). Some also occur as oxides (e.g., Fe₂O₃ – hematite). Chlorides and nitrates are less common as ores for these metals. Free metals are for least reactive metals like gold.
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Galvanisation is the process of applying a protective zinc coating to iron or steel to prevent rusting. Zinc is chosen because it is more reactive than iron. It corrodes preferentially (sacrificial protection), and it also forms a durable, adherent zinc oxide layer that provides a physical barrier. Zinc coating can be applied by hot-dip galvanising (immersing iron in molten zinc) or electrogalvanising. Coating with tin is called “tinning” (used for tin cans), while copper coating is copper plating.
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Copper is extracted from copper(I) sulphide (Cu₂S) by a process called smelting, which involves heating the ore in a limited supply of air. The reaction occurs in two stages: first, part of the Cu₂S is roasted to form Cu₂O: 2Cu₂S + 3O₂ → 2Cu₂O + 2SO₂. Then, the Cu₂O reacts with the remaining Cu₂S in the absence of air: 2Cu₂O + Cu₂S → 6Cu + SO₂. This is called auto-reduction or self-reduction because no external reducing agent is needed. The copper obtained is called blister copper.
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The removal of gangue (unwanted earthy impurities) from an ore is called enrichment, concentration, beneficiation, or dressing of the ore. This process increases the percentage of the desired metal-bearing compound in the ore. Common concentration methods include hydraulic washing (based on density differences), magnetic separation (based on magnetic properties), froth flotation (based on wetting properties), and leaching (based on chemical solubility). Refining is the final purification of the extracted metal. Roasting and reduction are steps in metal extraction.
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Rusting is the corrosion of iron to form hydrated iron(III) oxide (Fe₂O₃·xH₂O), a reddish-brown flaky substance. For rusting to occur, both oxygen (from air) and water (or moisture) must be present simultaneously. The overall reaction involves iron reacting with oxygen in the presence of water: 4Fe + 3O₂ + 2xH₂O → 2Fe₂O₃·xH₂O. If either oxygen or water is absent, rusting does not occur. This is why iron does not rust in dry air or in deoxygenated water. Salt accelerates rusting by acting as an electrolyte.
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Solder is a fusible alloy primarily composed of lead (Pb) and tin (Sn). The most common composition is 50% lead and 50% tin (or 60% tin and 40% lead). Solder has a low melting point (around 183°C for the eutectic composition), which is much lower than the melting points of its constituent metals. This property makes it ideal for joining (soldering) electrical components, wires, and metal sheets in plumbing and electronics. Iron and carbon form steel; copper and tin form bronze; copper and zinc form brass.
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Mercuric oxide (HgO) is a compound of mercury, a metal low in the reactivity series. When heated strongly, it undergoes thermal decomposition. The reaction is: 2HgO(s) → 2Hg(l) + O₂(g). This is a simple reduction reaction where the oxide decomposes into its constituent elements without needing an external reducing agent. Mercury is obtained as a liquid metal (it condenses), and oxygen gas is released. This principle is used to extract metals that are low in the reactivity series.
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Highly reactive metals (such as Na, K, Ca, Mg, Al) are at the top of the reactivity series. Their compounds (oxides, chlorides) are extremely stable and cannot be reduced by common reducing agents like carbon, carbon monoxide, or hydrogen. Therefore, they are extracted by electrolytic reduction (electrolysis) of their molten compounds. For example, sodium is obtained by electrolysis of molten NaCl; aluminium is obtained by electrolysis of molten Al₂O₃ dissolved in cryolite. The electrical energy provides the high reduction potential required.
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The purity of gold is measured in carats (karats). Pure gold is 24 carats (24K), meaning 24 out of 24 parts are gold (100% gold). 22-carat gold means that 22 parts out of 24 parts are gold, and the remaining 2 parts are other metals (usually copper, silver, or zinc). Therefore, 22K gold is 91.67% gold (22/24 × 100). Gold is alloyed with other metals because pure gold (24K) is too soft for making durable jewellery. Common carats are 22K, 18K (75% gold), and 14K (58.3% gold).
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When copper is exposed to moist air for a long time, it undergoes corrosion to form a green patina (coating). The green coating is primarily basic copper carbonate [CuCO₃·Cu(OH)₂], formed by the reaction of copper with atmospheric carbon dioxide, oxygen, and water: 2Cu + H₂O + CO₂ + O₂ → CuCO₃·Cu(OH)₂. This is seen on old copper roofs, statues (like the Statue of Liberty), and copper utensils. Copper oxide (black) forms on heating in air. Copper sulphide (black) forms in polluted air with hydrogen sulphide.
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In ores, metals are present in their oxidized form (as positive ions in compounds like oxides, sulphides, carbonates). To obtain the free metal, these positive ions must gain electrons to become neutral metal atoms. Gaining electrons is called reduction. The general principle of metal extraction is: Metal compound + Reducing agent → Free metal + By-product. For example: Fe₂O₃ + 3CO → 2Fe + 3CO₂ (iron is reduced). Thus, extraction of metals from their compounds is fundamentally a reduction process, not oxidation, neutralisation, or simply displacement.
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An amalgam is an alloy in which mercury (Hg) is one of the constituent metals. Mercury dissolves many other metals (like sodium, silver, gold, zinc, tin) to form pasty or liquid amalgams. Silver-tin amalgams are used in dental fillings. Sodium amalgam is used as a reducing agent in organic chemistry. Gold amalgam is used in gold extraction from ores. Brass (Cu+Zn), bronze (Cu+Sn), and solder (Pb+Sn) do not contain mercury.
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In electrolytic refining, a large block of the impure metal is made the anode (positive electrode), and a thin strip of the pure metal is made the cathode (negative electrode). They are placed in an electrolyte solution containing a soluble salt of the same metal. When electric current is passed, the impure metal anode dissolves (oxidizes), and pure metal is deposited on the cathode. Impurities either fall to the bottom as anode mud (for less reactive impurities) or remain in solution (for more reactive impurities).
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Silver tarnishes (turns black) when exposed to air over time. This black coating is silver sulphide (Ag₂S), not silver oxide. Silver reacts with trace amounts of hydrogen sulphide (H₂S) gas present in the air (from industrial pollution, volcanic activity, or decaying organic matter). The reaction is: 4Ag + 2H₂S + O₂ → 2Ag₂S (black) + 2H₂O. Silver oxide (Ag₂O) is brownish-black but not the primary tarnish product in normal air. Tarnish can be removed by polishing or by chemical reactions (e.g., with baking soda and aluminium foil).
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Gangue (also called matrix) is the term used for the unwanted, non-valuable earthy and rocky impurities present in an ore. These impurities include sand (silica), clay, soil, limestone, and other mineral debris. The ore contains two components: the valuable mineral (which contains the desired metal) and the gangue. The process of removing gangue from the ore is called concentration or beneficiation. A flux is a substance added during smelting to remove gangue as slag. An ore is a mineral from which metal can be profitably extracted.
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Pure iron (also called wrought iron) is relatively soft, malleable, and ductile, but it is not strong enough for most structural applications. It can be easily bent and does not hold a sharp edge. However, the main reason pure iron is not widely used is that it is too soft, not because it is costly (it is actually cheaper than many alloys) or solely because it corrodes (all iron corrodes). To improve its strength, hardness, and other properties, iron is alloyed with carbon and other elements to make steel and cast iron.
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Calcination is the process of heating a carbonate ore in a limited supply of air (or absence of air) to convert it into the metal oxide. When zinc carbonate (ZnCO₃, calamine) is calcined, it decomposes to form zinc oxide and carbon dioxide gas. The reaction is: ZnCO₃(s) → ZnO(s) + CO₂(g). The zinc oxide produced is then reduced to zinc metal using carbon (coke). Calcination is used for carbonate ores. Roasting (heating in excess air) is used for sulphide ores.
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The concentration of an ore (removal of gangue) relies on exploiting differences between the desired mineral and the gangue. These differences can be physical (e.g., density, magnetic susceptibility, wettability, particle size) or chemical (e.g., solubility in specific reagents). For example, hydraulic washing uses density differences; magnetic separation uses magnetic properties; froth flotation uses differences in wetting; leaching uses chemical solubility. No single technique works for all ores; the choice depends on the specific physical or chemical property difference between the ore mineral and gangue.
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Sodium is a highly reactive metal (alkali metal) at the top of the reactivity series. It cannot be extracted by chemical reduction because its compounds are very stable. Sodium is commercially produced by the electrolysis of molten (fused) sodium chloride (NaCl), often with added calcium chloride to lower the melting point. The reactions are: At cathode: Na⁺ + e⁻ → Na (liquid sodium metal); At anode: 2Cl⁻ → Cl₂ + 2e⁻ (chlorine gas). Electrolysis of aqueous NaCl produces sodium hydroxide and chlorine, not sodium metal, because water would react with sodium.
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Metals low in the activity (reactivity) series, such as copper (Cu), silver (Ag), gold (Au), and platinum (Pt), are called noble metals. They are chemically unreactive or very less reactive. They do not react with water, steam, or dilute acids. They do not tarnish easily in air. They are often found in the free (native) state in nature and can be extracted by simple heating of their oxides or by physical separation methods. Their low reactivity is due to their high reduction potentials, making them resistant to losing electrons.
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Zinc is a moderately reactive metal. After zinc oxide (ZnO) is obtained by calcination of ZnCO₃ or roasting of ZnS, it is reduced to zinc metal using carbon (coke) as a reducing agent. The reaction, carried out in a furnace at high temperature, is: ZnO(s) + C(s) → Zn(g) + CO(g). Zinc is volatile (boiling point 907°C) and distills out as vapour, which is then condensed. Carbon is a common reducing agent because it is cheap and readily available, and it has a high affinity for oxygen (forming CO or CO₂).
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In electrolytic refining, the setup consists of: Anode – impure metal block (positive electrode); Cathode – thin strip of pure metal (negative electrode); Electrolyte – solution of a soluble salt of the same metal. When electric current is passed, the impure metal anode dissolves (oxidizes) into the electrolyte as metal ions. These metal ions migrate to the cathode, where they gain electrons (are reduced) and deposit as pure metal on the cathode. Thus, the pure metal is collected at the cathode, while impurities either fall as anode mud or remain in solution.
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It is generally easier to extract metals from their oxide ores than from sulphide or carbonate ores. This is because the reduction of metal oxides (using carbon, carbon monoxide, or hydrogen) is a more straightforward process. Sulphide ores must first be converted to oxides by roasting (heating in air), which is an additional step. Carbonate ores must first be converted to oxides by calcination (heating in limited air). So, the extraction route for sulphides and carbonates involves first converting them to oxides, then reducing the oxides. Oxide ores can be reduced directly.
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Calcination is the process of heating an ore (typically a carbonate or hydroxide ore) strongly in a limited supply of air or in the absence of air. The purpose is to decompose the carbonate or hydroxide into the metal oxide and carbon dioxide or water. For example: ZnCO₃ → ZnO + CO₂; CaCO₃ → CaO + CO₂. Calcination also drives off volatile impurities. Roasting is heating a sulphide ore in excess air to convert it to an oxide. Refining purifies the metal. Reduction is the step that produces the free metal from the oxide.
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Stainless steel is an alloy of iron (Fe) with a minimum of about 10.5% chromium (Cr) and often contains nickel (Ni) as well. Chromium forms a thin, adherent, invisible chromium oxide (Cr₂O₃) layer on the surface, which prevents rusting (passivation). Nickel improves corrosion resistance, strength, and provides a shiny appearance. Some stainless steels also contain molybdenum, manganese, or carbon. Common types include 304 (18% Cr, 8% Ni) and 316 (16% Cr, 10% Ni, 2% Mo). Stainless steel does not rust like ordinary iron.
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Bronze is an alloy consisting primarily of copper (Cu) and tin (Sn). The typical composition is about 88% copper and 12% tin, though this can vary. Bronze is harder and more corrosion-resistant than pure copper. It was the first alloy developed by humans, giving its name to the Bronze Age. Bronze is used for statues (because it expands slightly before setting, capturing fine details), medals, ship propellers, bearings, and musical instruments (cymbals, bells). Copper and zinc form brass; lead and tin form solder; iron and nickel form various steel alloys.
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Metals at the top of the reactivity series (like K, Na, Ca, Mg, Al) are highly reactive and form very stable compounds (oxides, chlorides). Carbon, even at high temperatures, is not a strong enough reducing agent to remove oxygen from these stable compounds. The affinity of these metals for oxygen is greater than that of carbon. Therefore, carbon cannot reduce them. These metals are extracted by electrolytic reduction (electrolysis) of their molten compounds, which provides the high reduction potential needed. Carbon is used to reduce moderately reactive metals (Zn, Fe, Sn, Pb).
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An alloy is a homogeneous mixture (solid solution) of two or more metals, or a metal and a non-metal (like carbon in steel). The constituent elements are uniformly distributed at the atomic level, giving the alloy a single-phase structure. Alloys are not chemical compounds because they do not have fixed stoichiometric ratios; the proportions of the components can vary. They are also not heterogeneous mixtures because the components cannot be seen separately. Examples: brass (Cu + Zn), bronze (Cu + Sn), steel (Fe + C).
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Rust is the common name for the reddish-brown, flaky, powdery substance that forms on the surface of iron when it corrodes. Chemically, rust is hydrated iron(III) oxide (Fe₂O₃·xH₂O). It is formed by the reaction of iron with oxygen and water (moisture). The flaky nature of rust is problematic because it does not adhere tightly to the underlying metal; it flakes off, exposing fresh iron to further corrosion. “Iron oxide” is a general term that includes FeO, Fe₂O₃, and Fe₃O₄; rust is a specific hydrated form of iron(III) oxide.
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A reducing agent is a substance that donates electrons to another substance and gets oxidized itself. Aluminium is a highly reactive metal (it is high in the reactivity series) and has a very strong tendency to lose electrons (oxidize). This strong reducing power is the basis of the thermite reaction, where aluminium reduces metal oxides like iron(III) oxide: Fe₂O₃ + 2Al → 2Fe + Al₂O₃ + Heat. The heavier the metal, the more reactive it is not; reactivity is based on electron loss tendency, not weight or brittleness.
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Solder (an alloy of lead and tin) is primarily used for joining (soldering) electrical wires, electronic components on circuit boards, and plumbing joints. Its low melting point (around 183°C) allows it to be melted easily with a soldering iron, flowed into the joint, and then cooled to form a strong, electrically conductive, and leak-proof connection. Solder is not strong enough for bridges (steel is used), not used for jewellery (though gold/silver solder exists, it’s different), and not suitable for cooking vessels (would melt and toxic lead could leach).
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The thermite reaction is the reaction between aluminium powder and a metal oxide (usually iron(III) oxide, Fe₂O₃). The equation is: Fe₂O₃ + 2Al → 2Fe + Al₂O₃ + Heat. This reaction is highly exothermic, releasing a tremendous amount of heat (about 850 kJ per mole of Fe₂O₃). The temperature generated can reach 2500-3000°C, which is sufficient to melt the iron produced. This molten iron is used for welding railway tracks, joining heavy iron parts, and in incendiary devices. It is neither endothermic, slow, nor reversible.
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The famous Iron Pillar of Delhi (near Qutub Minar) has withstood corrosion for over 1600 years. This remarkable rust resistance is primarily due to the formation of a thin, protective, adherent layer of magnetite (Fe₃O₄, a mixed iron(II,III) oxide) on its surface. Unlike rust (Fe₂O₃·xH₂O), which is flaky and non-protective, magnetite is stable, dense, and forms a passivating layer. The high phosphorus content of the ancient wrought iron and the specific climatic conditions also contributed to this unique protection.
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In electrolytic refining, when the impure metal anode dissolves, any impurities that are less reactive than the metal and are insoluble in the electrolyte do not dissolve. These insoluble impurities, which may include precious metals like gold, silver, platinum, and other non-metallic residues, fall off the anode and settle at the bottom of the electrolytic cell. This collection is called anode mud (or anode sludge). Anode mud is often valuable because it contains precious metals that can be recovered. Gangue is the impurity in ore; slag is a molten by-product in smelting.
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The reaction between iron(III) oxide (Fe₂O₃) and aluminium powder is specifically known as the thermite reaction. It is a highly exothermic oxidation-reduction reaction where aluminium (a more reactive metal) reduces iron(III) oxide to molten iron. The equation is: Fe₂O₃(s) + 2Al(s) → 2Fe(l) + Al₂O₃(s) + Heat. The reaction is initiated by a high-temperature source like a magnesium ribbon. It is used for welding railway tracks (Thermit welding) and in some incendiary weapons. Roasting, calcination, and electrolytic reactions are different processes.
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In the electrolysis of molten sodium chloride (NaCl), the compound dissociates into Na⁺ and Cl⁻ ions. The positive Na⁺ ions migrate to the cathode (negative electrode), where they gain electrons to form sodium metal: Na⁺ + e⁻ → Na (reduction). The negative Cl⁻ ions migrate to the anode (positive electrode), where they lose electrons to form chlorine gas: 2Cl⁻ → Cl₂ + 2e⁻ (oxidation). Therefore, chlorine gas is liberated at the anode, and sodium metal is deposited at the cathode. A salt bridge is used in galvanic cells, not in this setup.
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Cinnabar is the principal ore of mercury (Hg). It is a sulphide ore with the chemical formula HgS (mercury(II) sulphide). Cinnabar has a characteristic bright red to reddish-brown color. Mercury is extracted from cinnabar by heating it in air (roasting): HgS + O₂ → Hg + SO₂. Since mercury is low in the reactivity series, its oxide decomposes easily, and the metal is obtained directly. Common ores: Zinc – zinc blende (ZnS) or calamine (ZnCO₃); Copper – chalcopyrite (CuFeS₂) or malachite; Iron – hematite (Fe₂O₃) or magnetite (Fe₃O₄).
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Metals that are low in the reactivity series (like mercury, silver, gold, and copper to some extent) have oxides that are unstable and decompose easily when heated. Their oxides can be reduced to the free metal by simple heating alone, without needing any external reducing agent like carbon or electrolysis. For example: 2HgO(s) → 2Hg(l) + O₂(g); 2Ag₂O(s) → 4Ag(s) + O₂(g). This is because these metals have a low affinity for oxygen, so the reverse reaction (decomposition) occurs readily at high temperatures.
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Roasting is the process of heating a sulphide ore strongly in the presence of excess air (oxygen). The purpose is to convert the metal sulphide into a metal oxide, and to remove the sulphur as sulphur dioxide (SO₂) gas. For example: 2ZnS + 3O₂ → 2ZnO + 2SO₂; 2PbS + 3O₂ → 2PbO + 2SO₂. Roasting is carried out below the melting point of the ore. Calcination is for carbonate/hydroxide ores in limited air. Smelting involves heating ore with a reducing agent. Refining purifies the extracted metal.
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In the electrolysis of molten sodium chloride (NaCl), the compound dissociates into Na⁺ and Cl⁻ ions. Reduction occurs at the cathode (the negative electrode). Sodium ions (Na⁺) gain electrons at the cathode to form sodium metal: Na⁺ + e⁻ → Na(l). This liquid sodium metal is either collected or floats to the top. Oxidation occurs at the anode (the positive electrode), where chloride ions lose electrons to form chlorine gas: 2Cl⁻ → Cl₂(g) + 2e⁻. Thus, sodium metal is deposited at the cathode.
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Electrolytic refining is the most widely used and versatile method for purifying metals obtained after extraction. It can produce metals of very high purity (99.9% to 99.99%). It is used for refining many important metals including copper, zinc, aluminium, silver, gold, and lead. In this method, impure metal is made the anode, and pure metal is deposited on the cathode. Distillation is used for volatile metals like zinc and mercury. Roasting and calcination are not refining methods; they are steps in extraction.
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Roasting is the process of heating a sulphide ore in excess air. When zinc sulphide (ZnS, zinc blende) is roasted, it reacts with oxygen to form zinc oxide and sulphur dioxide gas. The reaction is: 2ZnS(s) + 3O₂(g) → 2ZnO(s) + 2SO₂(g). The zinc oxide produced is then reduced to zinc metal using carbon: ZnO + C → Zn + CO. The SO₂ gas can be captured and used to produce sulphuric acid. Roasting does not directly produce zinc metal or ZnCO₃ (calcination produces ZnCO₃ → ZnO).
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The thermite reaction (Fe₂O₃ + 2Al → 2Fe + Al₂O₃ + heat) produces molten iron at about 2500-3000°C. This extremely hot molten iron is used for welding (joining) heavy iron and steel parts, particularly railway tracks, tram rails, and large iron structures. The process is called Thermit welding. The reaction is initiated locally, and the molten metal flows into the gap between the two pieces to be joined, fusing them strongly upon cooling. It is not used for extracting sodium (electrolysis is used), rust prevention (galvanisation/painting is used), or routine metal purification.
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Alloys generally have lower electrical conductivity and lower thermal conductivity than the pure metals from which they are made. This is because the presence of different atoms (different sizes and electronic structures) in the alloy creates irregularities and distortions in the regular crystal lattice. These irregularities scatter the free electrons (which are responsible for conductivity), reducing their mobility and thus lowering conductivity. For example, pure copper is an excellent conductor, but brass (Cu+Zn) and bronze (Cu+Sn) are poorer conductors. Steel conducts less than pure iron.
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Cinnabar is mercury(II) sulphide (HgS). When it is heated in air (roasting), it first reacts with oxygen to form mercury(II) oxide (HgO) and sulphur dioxide (SO₂): HgS + O₂ → HgO + SO₂. Mercury(II) oxide (HgO) is red or yellow. On further heating (since HgO is unstable at high temperature), it decomposes to form mercury metal and oxygen: 2HgO → 2Hg + O₂. So, the overall process gives mercury directly, but the first intermediate product formed is HgO. Mercury is low in the reactivity series, so its oxide decomposes easily.Close