Ores, Metallurgy and Alloys: Processes and Composition

The correct answer is B, Au, that is gold. Malleability is the property of being beaten into thin sheets and ductility the property of being drawn into wires, and gold possesses both to the highest degree of any metal: a single gram can be beaten into a sheet of about one square metre and drawn into a wire of great length, which is why gold leaf and gold wire have been made for centuries. Option A is wrong because sodium is so soft that it can be cut with a knife and is reactive enough to be stored under kerosene, so it cannot be drawn into a usable wire. Option C is wrong because cerium, a lanthanide used in lighter flints and alloys, is brittle and not valued for either property. Option D is wrong because mercury is the only metal that is liquid at room temperature, and a liquid can be neither beaten into sheets nor drawn into wires.

The correct answer is B, lead. Galena is lead sulphide, with the formula PbS, and is the principal ore from which lead is extracted; being a sulphide, it is concentrated by froth flotation and then roasted to the oxide before reduction. Option A is wrong because the ores of zinc are zinc blende, which is the sulphide ZnS, and calamine, which is the carbonate ZnCO3. Option C is wrong because tin comes from cassiterite or tin stone, the oxide SnO2, and is refined by liquation. Option D is wrong because mercury comes from cinnabar, the sulphide HgS, and is refined by distillation since it boils at a low temperature. The habit to build is to read the ending of the name: most sulphides of this group end in blende, pyrites, galena or cinnabar, while carbonates end in ite and are calcined rather than roasted.

The correct answer is B, sulphide ores. In froth flotation the powdered ore is agitated with water, a frothing agent such as pine oil and a collector. Sulphide particles are preferentially wetted by oil and are carried up with the froth, while the earthy gangue is wetted by water and settles at the bottom, so zinc blende and copper pyrites are concentrated this way. Option A is wrong because oxide ores are usually heavier than the gangue and are concentrated by hydraulic washing, as with haematite and tin stone. Option C is wrong because carbonate ores are not oil-wetted and are normally concentrated by washing before calcination. Option D is wrong because halide ores such as rock salt are soluble in water and are handled by dissolution and crystallisation. A depressant such as sodium cyanide can hold one sulphide back while another floats.

The correct answer is B. Calcination is the heating of an ore in the absence or a limited supply of air and below its melting point, so that water of hydration and carbon dioxide are driven off; it is therefore the step used for carbonate ores such as calamine and for hydrated ores such as bauxite. Option A is wrong because heating in a plentiful supply of air is roasting, which is applied to sulphide ores such as zinc blende, where sulphur escapes as sulphur dioxide. Option C is wrong because adding a reducing agent such as coke is the separate step of smelting or reduction, which comes after calcination or roasting. Option D is wrong because both calcination and roasting are deliberately carried out below the melting point; melting the ore is the business of smelting. Both steps share one aim, to obtain the oxide, which is the form easiest to reduce.

The correct answer is B. Zone refining rests on the fact that impurities are more soluble in a molten metal than in the same metal when solid. A narrow molten zone is created with a mobile heater and moved slowly along the rod, so the impurities travel with the melt and collect at one end, which is then cut off; the method gives the ultra-pure silicon, germanium and gallium used in semiconductors. Option A is wrong because a lower boiling point is the basis of refining by distillation, used for zinc, mercury and cadmium. Option C is wrong because the formation of a volatile carbonyl is the basis of the Mond process for nickel, where nickel carbonyl decomposes on stronger heating. Option D is wrong because oxidation into a porous cupel describes cupellation, used to separate silver from lead.

The correct answer is B, copper and zinc. Brass is made by alloying copper with zinc, which gives a metal harder than copper, easier to cast and machine, and resistant to ordinary corrosion, so it is used for utensils, taps and fittings and the bodies of musical instruments. Option A is wrong because copper with tin is bronze, the alloy of statues, coins and medals; bell metal is the same pair with a higher proportion of tin. Option C is wrong because copper with nickel is constantan, used for resistance wire and thermocouples, and copper with zinc and nickel together is German silver, which despite the name contains no silver. Option D is wrong because copper is a constituent of the aluminium alloy duralumin, but duralumin is an aluminium alloy used in aircraft and is not called brass.

The correct answer is B, nickel. German silver is an alloy of copper, zinc and nickel, and its name comes from its white silvery appearance, not from its composition, since it contains no silver at all; it is used for tableware, ornaments and electrical contacts. Option A is wrong precisely because of that trap: a candidate who reasons from the name will choose silver and lose the mark. Option C is wrong because tin with copper gives bronze, and tin with copper and zinc together gives gun metal, used for gears and steam fittings. Option D is wrong because lead with tin gives solder, and lead with antimony and tin gives type metal for printing. The list of deceptive names is itself worth learning: German silver has no silver, and nickel silver and Dutch metal are similarly named for their look.

The correct answer is B. Pure alumina melts at a very high temperature, so in the Hall and Heroult process it is dissolved in molten cryolite, sodium hexafluoroaluminate, usually with some fluorspar, which lowers the working temperature of the bath and makes the melt a far better conductor of electricity. Carbon anodes are then used and aluminium collects at the cathode. Option A is wrong because cryolite takes no part as a reducing agent; the reduction here is electrolytic, which is why aluminium cannot be obtained by smelting with coke like iron. Option C is wrong because the carbon anode is indeed slowly consumed by the oxygen liberated, but that is an unwanted side effect, not the purpose of adding cryolite. Option D is wrong because iron and silica impurities are removed earlier, during the Bayer leaching of bauxite with hot sodium hydroxide.

The correct answer is B, galvanisation. Galvanisation is the coating of iron or steel with zinc, usually by dipping it in molten zinc. Zinc is more reactive than iron, so even if the coating is scratched the zinc corrodes in preference to the iron beneath, a behaviour called sacrificial protection, which is why galvanised sheets and pipes last far longer than bare iron. Option A is wrong because tinning is coating with tin, used on the inside of food cans; tin is less reactive than iron, so a scratch in tin plate actually accelerates rusting. Option C is wrong because anodising is the thickening of the natural oxide film on aluminium, not a treatment for iron. Option D is wrong because chromium plating is done chiefly for appearance and hardness and gives no sacrificial protection to iron.

The correct answer is A, nickel. In the Mond process impure nickel is heated in a stream of carbon monoxide at a moderate temperature, forming the volatile compound nickel tetracarbonyl; the vapour is led away from the solid impurities and heated more strongly, whereupon it decomposes and deposits very pure nickel. Option B is wrong because titanium, along with zirconium, is refined by the van Arkel process, in which the metal forms a volatile iodide that decomposes on a hot tungsten filament. Option C is wrong because copper is refined electrolytically, with impure copper as the anode and pure copper as the cathode, and earlier by poling with green wood. Option D is wrong because zinc is refined by distillation, since it boils at a comparatively low temperature and the impurities do not.

The correct answer is B. Limestone decomposes in the furnace to calcium oxide, a basic flux, which combines with the acidic silica gangue of the ore to form calcium silicate, a fusible slag that floats on the molten iron and is drawn off separately. Option A is wrong because the reduction of iron oxide is done by coke and by the carbon monoxide it generates, not by limestone; this is the classic carbon reduction used for moderately reactive metals. Option C is wrong because the heat comes from the burning of coke in the blast of hot air, and the decomposition of limestone actually absorbs heat. Option D is wrong because pig iron does take up carbon from the coke, giving it the highest carbon content of the iron products, but limestone has no part in that; wrought iron carries the least carbon and steel an intermediate amount.

The correct answer is B, nichrome. Nichrome is an alloy of nickel, chromium and iron with a high electrical resistance and a high melting point, and it does not oxidise easily when red hot, so a nichrome wire can glow for years without burning away; that combination makes it the standard material for the heating elements of irons, toasters, geysers and hair dryers. Option A is wrong because invar is an alloy of iron and nickel chosen for its very low thermal expansion and used in pendulums and measuring tapes, where heating must not change the length. Option C is wrong because duralumin, an alloy of aluminium with copper, magnesium and manganese, is a light structural alloy used in aircraft. Option D is wrong because solder, an alloy of lead and tin, has a low melting point by design and is used to join wires.