General Science Mixed Quiz: Set 45

The correct answer is A, P. Phosphorus, sulphur, chlorine and argon all sit in the third period of the periodic table, so their electrons fill the same three shells. Moving left to right along a period, each element adds one more proton to the nucleus while the number of shells stays the same, so the nucleus pulls the outer electrons in more tightly and the atom gets smaller. Phosphorus stands furthest to the left of the four given elements, with the smallest nuclear charge, so it has the largest atomic radius. B is wrong because chlorine comes after both phosphorus and sulphur and is therefore smaller. D is wrong because sulphur lies between phosphorus and chlorine, so it is smaller than phosphorus. C is wrong because argon, a noble gas, is the last element of the period; its size is quoted as a van der Waals radius, which is measured differently and is not compared in this trend. Exam tip: size falls across a period, rises down a group.

The correct answer is C, Avogadro. The statement in the question is Avogadro's law, given by the Italian scientist Amedeo Avogadro in 1811. It says that equal volumes of all gases at the same temperature and pressure hold the same number of molecules, so the volume of a gas is directly proportional to the number of moles in it. This is why one mole of any gas occupies 22.4 litres at standard temperature and pressure, and why the number of particles in one mole, about 6.022 x 10 to the power 23, is named the Avogadro number after him. A is wrong because Charles law links the volume of a fixed mass of gas to its temperature at constant pressure. B is wrong because Boyle's law says that volume varies inversely with pressure at constant temperature. D is wrong because Gay-Lussac gave the law of combining volumes and the pressure-temperature law. Exam tip: Avogadro ties volume to the number of molecules, Boyle to pressure and Charles to temperature.

The correct answer is D, None of the above. None of the three substances listed slows a reaction down; the substance that does that is called a negative catalyst or an inhibitor. A catalyst changes the rate of a reaction without being used up in it, and it works by lowering the activation energy the reactants must cross. A positive catalyst speeds the reaction up, while a negative catalyst holds it back, the way glycerol slows the decomposition of hydrogen peroxide. A is wrong because a catalytic promoter is added to make the catalyst more active, so it raises the rate instead of lowering it. B is wrong because a homogeneous catalyst is one present in the same phase as the reactants, and it speeds the reaction up. C is wrong because a heterogeneous catalyst sits in a different phase, like iron in the Haber process, and it also speeds the reaction up. Exam tip: remember the pair - a promoter increases a catalyst's activity, a poison or inhibitor decreases it.

The correct answer is C, Stomata. Stomata are the tiny pores on the surface of a leaf through which a plant takes in carbon dioxide and gives out oxygen and water vapour, so the exchange of gases takes place through them. Each stoma is bounded by two bean-shaped guard cells that swell to open the pore and shrink to close it, and this is how the plant controls its water loss. Most stomata lie on the lower surface of a leaf, so less water is lost to direct sunlight. Transpiration also happens largely through these pores. A is wrong because the cell membrane controls what enters and leaves a single cell, not the gas exchange of the whole plant. B is wrong because the nucleus carries the genetic material and directs the activities of the cell. D is wrong because guard cells only open and close the pore, while the exchange itself happens through the stomatal opening. Exam tip: stomata do the gas exchange and transpiration, guard cells only open and close them.

The correct answer is C, Al2(SO4)3. A pH of 11.6 means the soil is strongly alkaline, so it has to be treated with something acidic, and aluminium sulphate is the only acidic compound in the list. Aluminium sulphate is a salt of a strong acid and a weak base. In water it hydrolyses and releases hydrogen ions, which pull the pH down towards neutral and let the roots absorb nutrients again. Gypsum and elemental sulphur are used in the same way on alkaline and saline soils. A is wrong because calcium hydroxide, or slaked lime, is a base and would push the pH even higher; it is spread on acidic soil instead. B is wrong because sodium chloride is a neutral salt, so it cannot change the pH, and extra salt harms the crop further. D is wrong because zinc hydroxide is also a base and is almost insoluble in water. Exam tip: acidic soil takes lime, alkaline soil takes gypsum or aluminium sulphate.

The correct answer is D, Colourless. Hydrogen is a colourless, odourless and tasteless gas, so none of the colours listed fits it. It is the lightest element, with one proton and one electron, and it is the most abundant element in the universe, although on the earth it is found mostly combined in water and in organic compounds. Hydrogen exists as the diatomic molecule H2, burns in air with a pale blue flame and gives only water as the product, which is why it is studied as a clean fuel. It is used to make ammonia by the Haber process and to harden vegetable oils into vanaspati. A is wrong because no common form of hydrogen is yellow. B is wrong because orange is the colour seen in sodium vapour lamps and not in hydrogen. C is wrong because none of the gases here is black; carbon in its soot form is black, but that is a solid. Exam tip: hydrogen, oxygen and nitrogen are all colourless and odourless gases.

The correct answer is D, Formation of an oxide layer. Copper turns green because its surface slowly reacts with the air and gets covered by a layer of corrosion products, and that layer is green. In moist air copper first forms a dark copper oxide; carbon dioxide and moisture then change it into basic copper carbonate, the green patina seen on old copper roofs, temple domes and the Statue of Liberty. The coating clings to the metal and shields what lies beneath, which is why copper does not waste away the way iron rusts. A is wrong because hydrogen embrittlement makes a metal brittle and crack-prone and does not colour its surface. B is wrong because copper does not take up nitrogen from the air, nitrogen being almost unreactive at ordinary temperatures. C is wrong because ultraviolet light does not react with copper to build a green coating. Exam tip: the green layer on copper is basic copper carbonate, and tamarind or lemon cleans copper vessels because its acid dissolves that layer.

The correct answer is B, In the same direction. In a class 3 lever the fulcrum is at one end, the effort is applied in the middle and the load sits at the far end. Since both the effort and the load lie on the same side of the fulcrum, the beam turns one way and they move in the same direction. The effort arm is shorter than the load arm, so the mechanical advantage is less than one and more effort is needed than the load; what such a lever gains is speed and a wider sweep of movement. The human forearm, a pair of sugar tongs, a fishing rod and a stapler work this way. A is wrong because effort and load move oppositely in a class 1 lever, where the fulcrum lies between them, as in a see-saw or scissors. C is wrong because a lever turns about a fixed point and its two ends never move at right angles to each other. D is wrong because the size of the load changes the force needed, not the direction of motion. Exam tip: class 1 has the fulcrum in the middle, class 2 the load, class 3 the effort.

The correct answer is C, Ethyl alcohol. Ethyl alcohol, or ethanol, dissolves resins, gums, dyes and oils and also evaporates quickly, so paints, varnishes and polishes made with it dry fast; this makes it the common solvent of the paint industry. Its formula is C2H5OH, and it is got by the fermentation of molasses or grain and, these days, from sugarcane for blending with petrol. The same property of being a good solvent makes ethanol the base of tincture of iodine, cough syrups and many tonics. Alcohol meant for industry is mixed with poisonous substances such as methanol and then coloured, giving denatured spirit, so that it is not drunk. A is wrong because methyl alcohol is highly poisonous and even a small quantity can cause blindness or death, so its use is restricted. B is wrong because n-propyl alcohol is costly and little used on this scale. D is wrong because iso-propyl alcohol serves mainly as a rubbing alcohol and a cleaning agent. Exam tip: ethanol is C2H5OH, and denatured spirit is ethanol made undrinkable with methanol.

The correct answer is C, 2 s. A simple pendulum about one metre long completes one full swing in roughly two seconds, and a pendulum of this length is called the seconds pendulum. The time period of a simple pendulum is T = 2 pi times the square root of L divided by g, where L is the length of the thread and g is the acceleration due to gravity, about 9.8 metres per second squared. Putting L = 1 metre gives T = 2 x 3.14 x the square root of 1 divided by 9.8, which comes to about 2.01 seconds. The period depends only on the length and on g, not on the mass of the bob or on how far it is pulled aside, so long as the swing is small. A is wrong because a 6 second period needs a thread about 9 metres long. B is wrong because a 4 second period needs a thread about 4 metres long. D is wrong because a 1 second period belongs to a thread of only about 25 centimetres. Exam tip: the period doubles only when the length becomes four times.

The correct answer is A, Carbon Dioxide. Dry ice is solid carbon dioxide. At ordinary pressure it does not melt into a liquid at all: it changes straight from solid to gas, a process called sublimation, at about minus 78.5 degrees Celsius. Because no liquid is left behind, the solid stays dry, which is how it got its name. It is used to keep ice cream, frozen food, vaccines and blood samples cold while they travel, and the thick white fog it makes on stage is only cold carbon dioxide gas condensing the moisture of the air. Option B is wrong because the oxides of nitrogen are gases, with nitrous oxide being the laughing gas used by dentists. Option C is wrong because oxidane is a systematic name for water, whose solid form is ordinary ice. Option D is wrong because sulphur dioxide is the pungent gas used as a preservative and bleach. Exam tip: dry ice is solid carbon dioxide and it sublimes, never melts.

The correct answer is A, low coefficient of friction. Teflon is the trade name of polytetrafluoroethylene, a polymer whose surface is one of the most slippery among solids, with a coefficient of friction of about 0.04. A bearing has to let a shaft turn with as little rubbing as possible, so this slipperiness is the reason Teflon is chosen; it also lets many such bearings work without any oil or grease. The polymer was found by accident by Roy Plunkett in 1938, resists almost every chemical and holds up to about 260 degrees Celsius, which is why it also coats non-stick cookware and is wound as sealing tape on pipe threads. B is wrong because Teflon is a poor conductor of heat, so it is not chosen to carry heat away. C is wrong because saving space depends on the design of the bearing, not on the material. D is wrong because only the first reason holds. Exam tip: Teflon is PTFE, the non-stick polymer with the lowest friction.

The correct answer is C, the liver. It is the largest gland as well as the heaviest internal organ of the human body, it lies in the upper right part of the abdomen, and its chief contribution to digestion is bile, which is stored in the gall bladder and emulsifies fat. Option A is wrong because the pancreas, although important as the only gland that is both exocrine and endocrine, is much smaller and weighs less than a hundred grams. Option B is wrong because the thyroid, the largest endocrine gland of the body, is still far smaller than the liver; the question asks for the largest gland of any kind. Option D is wrong because the parotid is only the largest of the three pairs of salivary glands, which is a much narrower title. A common trap in papers is to offer the thyroid, so read whether the question says gland or endocrine gland.

The correct answer is D, 2 gram of carbon. Three grams of hydrogen, 48 grams of oxygen and 42 grams of nitrogen each come to 1.5 moles, while two grams of carbon is only about 0.17 mole, so carbon holds a different number of particles. The number of moles is found by dividing the given mass by the molar mass, and equal moles always mean an equal number of particles. Hydrogen gas is H2 with a molar mass of 2, so 3 divided by 2 is 1.5 moles. Oxygen gas is O2 with a molar mass of 32, so 48 divided by 32 is 1.5 moles. Nitrogen gas is N2 with a molar mass of 28, so 42 divided by 28 is 1.5 moles. Carbon has an atomic mass of 12, so 2 divided by 12 is about 0.17 mole. A is wrong because 3 grams of hydrogen is 1.5 moles. B is wrong because 48 grams of oxygen is also 1.5 moles. C is wrong because 42 grams of nitrogen is 1.5 moles too. Exam tip: convert every mass to moles first.

The correct answer is C, the pulmonary artery. An artery is defined by direction, not by the kind of blood it holds: it carries blood away from the heart. The pulmonary artery leaves the right ventricle carrying the deoxygenated blood that has come back from the body, and takes it to the lungs to be loaded with oxygen, so it is the single exception to the rule that arteries carry oxygenated blood. Its partner exception is the pair of pulmonary veins, the only veins that carry oxygenated blood. Option A is wrong because the aorta leaves the left ventricle with fully oxygenated blood and is the largest artery of the body. Option B is wrong because the coronary arteries branch off the aorta to feed the heart muscle itself and therefore carry oxygenated blood; their blockage causes a heart attack. Option D is wrong because the carotid arteries carry oxygenated blood up to the head and brain.

The correct answer is D, Gustatory receptors. The receptors that pick up taste are called gustatory receptors, and they sit inside the taste buds on the tongue. They respond only to substances dissolved in saliva, which is why a dry tongue tastes little, and they report five basic tastes, sweet, sour, salty, bitter and umami. From the tongue the signal travels along cranial nerves to the brain, where taste and smell together give food its flavour. A is wrong because sensory receptor is the broad name for every receptor of the body, including those for light, sound, touch and pressure. B is wrong because chemoreceptor is the wider class that covers both taste and smell, so it does not name the taste receptor in particular. C is wrong because olfactory receptors lie in the lining of the nose and detect smell. Exam tip: gustatory means taste, olfactory means smell, auditory means hearing.

The correct answer is C, Vitamin C. Only vitamin C and the eight vitamins of the B complex dissolve in water; the rest dissolve in fat. Vitamin C, chemically ascorbic acid, is not stored in the body, so the extra amount leaves in the urine and the diet must supply it every day through amla, guava, citrus fruits and green vegetables. A shortage causes scurvy, with bleeding gums and slow healing of wounds, and the vitamin is easily destroyed by heat during cooking. A, B and D are wrong because vitamins D, A and K are fat soluble, are stored in the liver and fatty tissue, and can build up to harmful levels if taken in excess. Their deficiencies give rickets, night blindness and poor clotting of blood in that order. Exam tip: water soluble means C and the B group, so the fat soluble four are A, D, E and K.

The correct answer is D, direct change of a substance from a solid to a gas state. Sublimation is the change from solid to vapour without passing through the liquid stage. It happens when particles of the solid gain enough energy to escape straight into the gas phase, and the reverse change, gas to solid, is called deposition. Camphor, naphthalene balls, iodine, ammonium chloride and solid carbon dioxide, or dry ice, sublime on heating, which is why mothballs shrink away without leaving a wet patch. Chemists use sublimation to purify such solids by separating them from non-volatile impurities. Option A is wrong because a gas changing into a liquid is condensation. Option B is wrong because a gas changing straight into a solid is deposition. Option C is wrong because a liquid changing into a gas is vaporisation or evaporation. Exam tip: solid to gas is sublimation, gas to solid is deposition, and camphor is the stock example.

The correct answer is B, Mercury. Cinnabar is mercuric sulphide, HgS, and the formula itself gives the answer away because Hg is the symbol of mercury. It is the chief ore of mercury, a brick red mineral that is roasted in air to free the metal, and the same mineral ground fine was the old pigment vermilion. Mercury is the only metal that is liquid at room temperature, which is why it was long used in thermometers and barometers. A is wrong because aluminium is obtained from bauxite, its oxide ore. C is wrong because gold occurs mostly in the native state, and calaverite is a minor gold ore. D is wrong because silver comes from ores such as argentite, which is silver sulphide. Exam tip: read the formula first - Hg in HgS means mercury, and remember bauxite for aluminium and argentite for silver.

The correct answer is B, are long chain molecules held together by secondary bonds. In a thermoplastic the polymer exists as long linear or lightly branched chains, and one chain is held to the next only by weak secondary forces such as van der Waals attraction. Heat gives enough energy to loosen those weak links, so the material softens, can be moulded into a new shape and hardens again on cooling, and the cycle can be repeated, which is why thermoplastic waste is recyclable. Polyethylene, polypropylene, PVC, polystyrene and nylon all belong to this group. A is wrong because a three-dimensional network of strong primary bonds describes a thermosetting plastic such as bakelite, which sets once and cannot be remoulded. C is wrong because the ability to deform plastically rises with temperature instead of falling. D is wrong because the secondary bonds do break as thermal energy rises, and that is exactly why the plastic softens. Exam tip: thermoplastic means weak secondary bonds and remoulding, thermosetting means cross-linked primary bonds and a shape set for good.