Lesson 8: Metallurgy

Rule 1: Physical properties of metals — Metals are mostly solids; mercury and gallium are liquid at room temperature. They have luster (चकाकी), which decreases on exposure to oxygen, moisture and some reactive gases. They are ductile (तन्य, drawn into wires), malleable (वर्धनीय, beaten into sheets) and good conductors of heat and electricity. Metals are generally hard, but alkali metals of group 1 (lithium, sodium, potassium) are soft and can be cut with a knife. Tungsten has the highest melting point (3422 °C). Metals that make a sound when struck are sonorous (नादमय).

In the figure : In the figure the properties of metals and non-metals.

• Easy : Easy: copper is drawn into thin wires because it is ductile; aluminium is beaten into foil because it is malleable.
• Medium : Medium: school bells are made of metal because it is sonorous.
• Exam-type : Exam: why are sodium and potassium kept under kerosene, and why can they be cut with a knife? They are soft, highly reactive alkali metals.

Rule 2: Nonmetals and ions — Nonmetals are solids or gases (bromine is a liquid); they have no luster (iodine crystals are shiny), are not hard (diamond, the hardest natural substance, is an exception), have low melting and boiling points, and are bad conductors (graphite is a good conductor of electricity). Diamond is a bad conductor of electricity but a good conductor of heat. Metals lose electrons and form positive ions, so they are electropositive (विद्युतधन); nonmetals gain electrons and form negative ions, so they are electronegative (विद्युतऋण).

• Easy : Easy: graphite in a pencil lead conducts electricity, though carbon is a nonmetal.
• Medium : Medium: sodium (2,8,1) loses one electron to form Na⁺; chlorine (2,8,7) gains one electron to form Cl⁻.
• Exam-type : Exam: diamond and graphite are both carbon, yet diamond is hard and an insulator, while graphite is soft and a good conductor of electricity.

Spot the difference

Rule 1: Reactions of metals — With oxygen: metals form oxides. Sodium reacts with air even at room temperature (4Na + O₂ → 2Na₂O) and catches fire, so it is kept in kerosene. Oxides like Na₂O and MgO react with water to give the alkalis NaOH and Mg(OH)₂. With water: sodium and potassium react rapidly and vigorously, releasing hydrogen; calcium reacts slowly and floats; aluminium, iron and zinc do not react with cold or hot water but react with steam to form oxides and hydrogen. With dilute acids the order of reactivity is Mg > Al > Zn > Fe. Aqua regia (conc. HCl : conc. HNO₃ = 3 : 1) dissolves gold and platinum.

• Easy : Easy: Mg + 2HCl → MgCl₂ + H₂ is faster than Fe + 2HCl, since Mg is more reactive.
• Medium : Medium: calcium floats on water because the hydrogen bubbles stick to it, while sodium reacts rapidly and may catch fire.
• Exam-type : Exam: write the balanced equation of 2Na + 2H₂O → 2NaOH + H₂ and say why sodium is kept in kerosene.

In the figure : In the figure the reactivity series.

Rule 2: Displacement and the series — If metal A displaces metal B from its salt solution, A is more reactive than B. Iron displaces copper from copper sulphate, so iron is more reactive than copper. The reactivity series lists K, Na, Li, Ca, Mg, Al, Zn, Fe, Sn, Pb, Cu, Hg, Ag, Au from the most to the least reactive; metals are divided into highly, moderately and less reactive.

In the figure : In the figure iron displaces copper, so iron is more reactive.

• Easy : Easy: Zn + CuSO₄ → ZnSO₄ + Cu happens, so zinc is more reactive than copper.
• Medium : Medium: copper placed in zinc sulphate solution shows no change, because copper is less reactive and cannot displace zinc.
• Exam-type : Exam: arrange Cu, Mg, Fe, Ag in decreasing order of reactivity: Mg > Fe > Cu > Ag.

Spot the difference

Rule 1: Nonmetals and the octet — Noble gases (helium, neon, argon) are chemically inert because their outermost shell is complete. Metals attain the noble gas octet by losing electrons, nonmetals by gaining electrons. Nonmetals usually form acidic oxides (CO₂, SO₂); CO is a neutral oxide. Nonmetals generally do not react with water or dilute acids, but halogens do (Cl₂ + H₂O → HOCl + HCl).

• Easy : Easy: Mg (2,8,2) loses 2 electrons to become Mg²⁺ with octet 2,8.
• Medium : Medium: oxygen (2,6) gains 2 electrons to become O²⁻ (2,8); magnesium oxide MgO forms from Mg²⁺ and O²⁻.
• Exam-type : Exam: CO₂ and SO₂ are acidic oxides of nonmetals, and CO is neutral; metal oxides are usually basic.

Rule 2: Properties of ionic compounds — They are formed from a cation and an anion held by an electrostatic force called the ionic bond (आयनिक बंध). They are electrically neutral, crystalline, hard and brittle, with high melting and boiling points (NaCl 801 °C, MgO 2852 °C). They dissolve in water but not in kerosene or petrol. They do not conduct electricity in the solid state but conduct in the molten state and in aqueous solution, because the ions are mobile; so they are called electrolytes (विद्युत अपघट्य).

In the figure : In the figure sodium and chlorine form an ionic bond by transfer of an electron.

• Easy : Easy: solid salt does not light a bulb, but salt solution does, because the ions move in solution.
• Medium : Medium: NaCl melts at 801 °C and MgO at 2852 °C; MgO has stronger attraction (2+ and 2−), so a higher melting point.
• Exam-type : Exam: why can't solid NaCl conduct electricity but molten NaCl can? In the solid, ions are fixed; in the melt they move freely.

Spot the difference

Rule 1: Minerals, ores and concentration — Reactive metals occur in combined form (oxides, carbonates, sulphides, nitrates); silver, gold and platinum occur in the free state. Metal compounds found with impurities are minerals (खनिजे); those from which a metal can be separated economically are ores (धातुके); impurities like soil and sand are gangue (खनिज मळी). Concentration of ores uses gravitation methods (Wilfley table, hydraulic separation), magnetic separation (cassiterite), froth floatation (sulphide ores like ZnS and CuFeS₂, with pine oil) and leaching (bauxite leached with NaOH or Na₂CO₃).

In the figure : In the figure four ways to concentrate an ore.

• Easy : Easy: ore = mineral from which a metal can be taken out profitably; gangue = sand and soil mixed in.
• Medium : Medium: froth floatation is used for sulphide ores because sulphide particles stick to the oil froth and float, leaving the gangue behind.
• Exam-type : Exam: which method for which ore – cassiterite: magnetic; ZnS: froth floatation; bauxite: leaching.

Rule 2: Extraction by reactivity — Reactive metals (Na, Ca, Mg) are obtained by electrolysis of their molten chlorides: the metal deposits at the cathode (reduction) and chlorine is liberated at the anode (oxidation). Aluminium comes from bauxite (30–70% Al₂O₃) by Bayer's or Hall's process, followed by electrolysis of alumina with cryolite and fluorspar to lower the melting point to about 1000 °C; graphite anodes are replaced from time to time.

In the figure : In the figure the extraction method depends on reactivity.

• Easy : Easy: at the cathode Na⁺ + e⁻ → Na, and at the anode 2Cl⁻ → Cl₂ + 2e⁻.
• Medium : Medium: 1000 kg of bauxite with 60% Al₂O₃ contains 1000 × 60 ÷ 100 = 600 kg of alumina.
• Exam-type : Exam: why is cryolite added in the electrolysis of alumina? It lowers the melting point (to about 1000 °C) and saves energy.

Rule 3: Moderately reactive metals — Moderately reactive metals (Fe, Zn, Pb, Cu): sulphide ores are heated in air (roasting, भर्जन) and carbonate ores in limited air (calcination, निस्तापन) to give oxides, which are reduced by carbon. The thermit reaction (Fe₂O₃ + 2Al → 2Fe + Al₂O₃) is used to weld rails. Copper is obtained from Cu₂S by heating in air. Impure metals are refined by electrolysis.

• Easy : Easy: roasting ZnS: 2ZnS + 3O₂ → 2ZnO + 2SO₂.
• Medium : Medium: calcination of ZnCO₃: ZnCO₃ → ZnO + CO₂; then reduction: ZnO + C → Zn + CO.
• Exam-type : Exam: roasting is heating a sulphide ore in plenty of air; calcination is heating a carbonate ore in limited air.

Spot the difference

Rule 1: Corrosion — Iron forms rust (Fe₂O₃·H₂O) in moist air. Copper turns green due to copper carbonate (patination). Silver turns black due to silver sulphide. Aluminium forms a thin protective oxide layer that stops further corrosion.

• Easy : Easy: a rusted iron nail is weaker and brown; the rust flakes off.
• Medium : Medium: old brass or copper vessels turn green; this is copper carbonate formed by air and moisture.
• Exam-type : Exam: why does aluminium not corrode badly? Its thin oxide layer protects the metal below.

Rule 2: Prevention and alloys — Keep the metal away from air with paint, oil, grease or varnish (short-term protection). Galvanizing (जस्तलेपन): a zinc coating on iron; zinc is more electropositive than iron. Tinning ('kalhaee'): tin on copper and brass vessels. Anodization: the metal (aluminium, copper) as anode forms a strong oxide layer. Electroplating: a less reactive metal coated on a more reactive one, for example silver-plated spoons. Alloys (मिश्रधातू) are homogeneous mixtures of a metal with other metals or nonmetals: bronze is 90% copper and 10% tin; stainless steel is mostly iron (about 74%) with chromium (18%) and the rest nickel or carbon (8%). An alloy containing mercury is an amalgam.

In the figure : In the figure ways to protect metals from corrosion.

• Easy : Easy: 200 g of bronze has 200 × 90 ÷ 100 = 180 g copper and 20 g tin.
• Medium : Medium: a 500 g stainless steel piece has 500 × 18 ÷ 100 = 90 g chromium and about 500 × 74 ÷ 100 = 370 g iron.
• Exam-type : Exam: galvanized iron is protected even if scratched, because zinc is more reactive and corrodes first, sparing the iron.

Spot the difference