General Science Mixed Quiz: Set 65

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, Concrete. Concrete is the building material made by mixing cement with coarse aggregate such as gravel, fine aggregate such as sand, and water.

The cement and water react in a process called hydration, which sets into a hard mass and binds the aggregate together. A common ratio for ordinary work is one part cement, two parts sand and four parts gravel, written as 1:2:4. Concrete is strong in compression but weak in tension, so steel bars are embedded in it to make reinforced cement concrete, the material of most modern buildings, bridges and dams.

Option A, mortar, is cement or lime with sand and water only, with no coarse aggregate, and is used to join bricks. Option C, slurry, is any thin, pourable mix of a solid in a liquid. Option D, a moderator, is the material that slows neutrons in a nuclear reactor.

Exam tip: mortar has no gravel; concrete has gravel; adding steel gives RCC.

The correct answer is A, Hydrogen. Its calorific value is about 150 kilojoules per gram, the highest of any known fuel, roughly three times that of petrol, and its only combustion product is water vapour, which makes it a clean fuel as well. For these reasons hydrogen is called the fuel of the future, although it is costly to produce and hard to store and transport safely because it is a very light gas that must be kept at high pressure or very low temperature. B is wrong because liquefied petroleum gas, a mixture of butane and propane, gives about 55 kilojoules per gram. C is wrong because petrol gives about 45 kilojoules per gram, the same order as kerosene and diesel. D is wrong because coal gives only about 25 to 33 kilojoules per gram, depending on its grade, and also leaves ash and smoke behind.

The correct answer is A, Petiole. The petiole is the stalk that joins a leaf blade to the stem; it belongs to the leaf, not to the flower, and a leaf without one is called sessile. A typical flower is built of four whorls borne on the thalamus. The calyx is the outermost ring of sepals, which protect the bud. Inside it the corolla of petals attracts pollinators. Then comes the androecium, the male whorl, whose units are the stamens, each with an anther on a filament. At the centre is the gynoecium, the female whorl, whose unit is the pistil or carpel, made of stigma, style and ovary. Option B is wrong because the pistil is the female part of the flower. Option C is wrong because the stamen is its male part. Option D is wrong because sepals form the calyx, the flower's outermost whorl. Exam tip: the four floral whorls are calyx, corolla, androecium and gynoecium; the petiole is a part of the leaf.

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 A, Two dry surfaces. Coulomb friction, also called dry friction, is the resistance that acts when two solid surfaces touch each other with no lubricant between them. Charles-Augustin de Coulomb set out its rules: the friction force is proportional to the normal reaction, it does not depend on the apparent area of contact, and it is very nearly independent of the sliding speed. Dry friction covers both the static friction that must be beaten before motion can start and the kinetic friction that acts once sliding has begun, and the limiting static value is the larger of the two. Option B is wrong because friction between lubricated surfaces is called fluid or film friction and obeys the laws of viscosity instead. Option C is wrong because relative motion describes kinetic friction, which is only one part of Coulomb friction. Option D is wrong because the resistance between a solid and a liquid is viscous drag. Exam tip: Coulomb friction means dry friction, proportional to the normal reaction and independent of the contact area.

The correct answer is B, butane and propane. Liquefied petroleum gas is obtained during the refining of petroleum and from natural gas processing, and it is chiefly a mixture of butane and propane, which liquefy under moderate pressure at ordinary temperature and so can be stored in a cylinder. Because the gas has no smell of its own, a strong-smelling substance called ethyl mercaptan is added so that a leak can be detected. A is wrong because methane with some ethane is the composition of natural gas, which is supplied as compressed natural gas or through pipelines, not of cylinder gas. C is wrong because a mixture of carbon monoxide and hydrogen is water gas, an industrial fuel, and coal gas has a similar origin. D is wrong because ethylene and acetylene are industrial gases, acetylene being used with oxygen for welding because of its very hot flame.

The correct answer is D, Charles’s law. Charles’s law says that at constant pressure the volume of a fixed mass of gas is directly proportional to its absolute temperature, so V by T stays constant. Jacques Charles studied this in 1787, and Joseph Gay-Lussac published the quantitative form in 1802. Written as a ratio it gives V1 by T1 equals V2 by T2, with the temperature always taken in kelvin. Extending the volume-temperature graph backwards to zero volume gives absolute zero, minus 273.15 degrees Celsius, which is how the kelvin scale was fixed. A is wrong because Boyle’s law relates pressure and volume at constant temperature, with PV constant. B is wrong because Dalton’s law of partial pressures adds the pressures of the gases in a mixture. C is wrong because Avogadro’s law links equal volumes of gases to equal numbers of molecules. Exam tip: keep the three apart - Boyle for P and V, Charles for V and T, Gay-Lussac for P and T.

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 A, Crown Gall. Crown gall is caused by the bacterium Agrobacterium tumefaciens, which enters through a wound near the base, or crown, of the stem and makes the plant grow rough tumour-like swellings there. The bacterium works by pushing a piece of its own Ti plasmid into the plant's cells and forcing them to divide, and that natural trick is why laboratories use it as a vector to carry new genes into plants. Grapevine, apple, rose, walnut and stone fruits suffer the most. Option B is wrong because mosaic diseases, such as tobacco mosaic, are caused by viruses that mottle the leaves in light and dark patches. Option C is wrong because smut of wheat, bajra and sugarcane is a fungal disease that fills the ear with black powdery spores. Option D is wrong because white blister, or white rust, of crucifers such as mustard is also fungal and raises white blisters on leaves and stems. Exam tip: bacterial — crown gall, citrus canker, bacterial blight of rice; viral — mosaic, leaf curl; fungal — smut, rust, blight.

The correct answer is D, Anthracite. Coal forms by the slow burial and compression of plant matter, and the longer and deeper the process goes the more carbon is left behind as moisture and volatile matter are driven off. Anthracite is the final and hardest stage, with the highest carbon content and the highest calorific value, and it burns with a short blue flame and almost no smoke, which makes it the best quality coal. A is wrong because peat is the first stage, barely coal at all, with a very high moisture content and the lowest carbon. B is wrong because lignite or brown coal is the next stage, soft and still moist. C is wrong because bituminous coal, the commonest variety and the one used to make coke for metallurgy, lies between lignite and anthracite. Remember the rising order: peat, lignite, bituminous, anthracite.

The correct answer is B, 1.5055 × 10²³. One mole of any gas occupies 22.4 litres at STP, so 5.6 litres is 5.6 divided by 22.4, that is one quarter of a mole. A mole contains 6.022 × 10²³ particles, the Avogadro number, so a quarter of a mole of oxygen contains 6.022 × 10²³ divided by four, which works out to 1.5055 × 10²³ molecules. Note that the question asks for molecules; since oxygen is diatomic, the number of atoms would be twice this figure. Option A is wrong because 6.022 × 10²³ is the count for a full mole, that is for 22.4 litres. Option C is wrong because 2.24 × 10²³ comes from confusing the molar volume 22.4 with the particle count. Option D is wrong because 3.011 × 10²³ is half a mole, which would be 11.2 litres. Exam tip: at STP, 22.4 litres equals one mole equals 6.022 × 10²³ molecules, so divide the given volume by 22.4 first.

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 D, stomata. The tiny pores on the surface of a leaf, found mostly on its lower side, are called stomata.

Each stoma is a slit guarded by two bean shaped guard cells, which swell to open it and shrink to close it. Through these pores the plant takes in carbon dioxide for photosynthesis and gives out oxygen, and water vapour escapes through them in the process called transpiration. Having them on the shaded lower surface reduces the loss of water. Floating plants such as the water lily carry their stomata on the upper surface instead, and fully submerged plants have almost none.

A is wrong: pore is only the general word for an opening and is not the botanical term. B is wrong: opening is likewise a plain description, not a name. C is wrong: cilia are tiny hair like structures that help some cells and organisms move, and they are not found on leaves.

Exam tip: stomata are leaf pores guarded by guard cells, mostly on the lower surface, used for gas exchange and for transpiration.

The correct answer is B, oil floats on water and the fire spreads further. Petrol and kerosene are lighter than water and do not mix with it, so water poured on an oil fire sinks below the burning layer and carries it outwards, enlarging the fire instead of putting it out. Such fires are smothered with carbon dioxide, foam or dry chemical powder, which cut off the supply of oxygen. A is wrong because ignition temperature is a fixed property of a substance and no extinguisher changes it; water works on ordinary fires by lowering the temperature of the fuel below that fixed value. C is wrong because water does not react chemically with petroleum fuels at all; that behaviour belongs to metals such as sodium and to burning magnesium. D is wrong because water does absorb a great deal of heat, having a high specific heat and a high latent heat of vaporisation, which is precisely why it works so well on wood and paper fires.

The correct answer is A, 1 and 4. Hooke's law holds only as far as the limit of proportionality, and that limit is either equal to or lower than the elastic limit, so statements 1 and 4 are the correct pair. Hooke's law says stress is proportional to strain, which is the straight part of the stress-strain graph; the point where the curve leaves the straight line is the limit of proportionality. A little beyond it the material still springs back to its original shape, and the last point at which it does so is the elastic limit. For most metals the two points lie very close together and for an ideal material they coincide. B is wrong because statement 2 extends the law past the straight part of the graph, where stress and strain are no longer proportional. C and D are wrong because statement 3 says always less, while the two limits may also be equal. Exam tip: order the four points as proportionality limit, elastic limit, yield point, ultimate stress.

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 C, it is heavier than air, covers the fire and cuts off the oxygen supply. Carbon dioxide is about one and a half times as dense as air, so when released it forms a blanket over the burning material and separates it from atmospheric oxygen, removing one side of the fire triangle. It is a poor conductor of electricity and leaves no residue, so it does not damage electrical equipment or electrocute the user, unlike water. Its sudden expansion from the cylinder also cools the fuel, which helps. A is wrong because carbon dioxide is denser than air, not lighter; a lighter gas would drift away and be useless. B is wrong because carbon dioxide does not react with ordinary fuels; it works physically, by exclusion of oxygen. D is wrong because ignition temperature is a fixed property of a substance and cannot be raised by any extinguisher.

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 C, ignition temperature. It is the lowest temperature to which a combustible substance must be heated before it begins to burn, and it is a fixed property of that substance; substances with a very low ignition temperature, such as petrol, alcohol and liquefied petroleum gas, are called inflammable substances and are stored with care. A is wrong because the boiling point is the temperature at which a liquid turns to vapour at a given pressure and has nothing to do with catching fire. B, flash point, is the closest distractor and is a related but different idea: it is the lowest temperature at which the vapour above a liquid fuel gives a momentary flash when a flame is brought near, without the liquid itself continuing to burn. D is wrong because the calorific value is the heat released on burning a unit mass of fuel, a measure of energy and not of temperature.