Combined Science Paper 2 Specimen 2018

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1. Athe thin wall stops any dust or bacteria in the inhaled air from reaching the bloodstream below it.
2. Bthe wall is strong enough to hold the alveolus open when the air pressure inside the lung falls.
3. Cthe thin wall keeps the air inside the alveolus at the temperature of the blood flowing past it.
4. Doxygen and carbon dioxide have a very short distance to diffuse across, so the exchange is quick.

1. Athe roots take up water only in the middle of the day, so there is nothing for the leaves to lose in the early morning or the evening.
2. Bevaporation from the leaf rises as the sun climbs and is greatest when the sun is highest, then falls away as the sun sets and the stomata close.
3. Cthe stomata open at sunset and close again at sunrise, so the leaf gives out most of its water vapour during the cool hours of darkness rather than during the day.
4. Dthe plant runs out of water by the middle of the afternoon and has to wait for the soil to be watered again before it can transpire.

1. Aa new plant grows from a stem, root or leaf of the parent plant, with no seed and no fertilisation involved.
2. Ba seed formed inside the fruit is dispersed by wind, water or animals and germinates into a new plant some distance away from the parent that produced it.
3. Cthe male and female sex cells of two different parent plants fuse to form a zygote inside the ovule.
4. Dpollen from the anther of one flower is carried to the stigma of another flower on the same plant.

1. Asugar cane, whose cut stem lengths each carry buds that sprout into new canes.
2. Bsunflower, whose oily seeds are sown directly into the prepared land at the start of the rains.
3. Cmaize, whose grains are shelled off the cob and planted in rows once the soil is warm enough.
4. Dgroundnut, whose pods are lifted, dried and shelled and the kernels planted the following season.

1. Athe plants will be able to pollinate one another more easily, since they all flower at exactly the same time.
2. Bthe plants need no water for their first season, because a sucker carries all the water the new plant will use.
3. Cevery plant is genetically identical to the parent, so the high yield and the disease resistance are passed on unchanged.
4. Dthe plants raised this way vary a great deal from one to the next, so whatever weather the season brings, at least some of them will be suited to it and give a crop.

1. Athe tuber carries a large store of food that feeds the young shoot until it can make its own.
2. Bthe tuber needs neither water nor oxygen, so it can be planted at any time of the year.
3. Cthe tuber grows a much deeper and wider root system than a seedling does in its very first week, so it reaches water far below the level the seedling can draw on.
4. Dthe tuber has already been fertilised, so it begins growing before it is even planted.

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1. Amass number 35 and proton number 18, since an isotope has one more particle in its nucleus.
2. Bmass number 37 and proton number 19, since both figures rise together from one isotope to the next.
3. Cmass number 33 and proton number 16, since an isotope has one less of each kind of particle.
4. Dmass number 37 and proton number 17, since an isotope has the same protons but more neutrons.

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1. Acontains carbon and hydrogen only, and no other element at all.
2. Bcontains carbon, hydrogen and oxygen, which are the three elements found in every fuel.
3. Ccontains carbon combined with any non metal, including chlorine and nitrogen.
4. Dcontains long chains of carbon atoms and burns to give carbon dioxide and water.

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1. Ait contains fewer carbon atoms than any other hydrocarbon in its homologous series.
2. Bthe double bond can open and take on further atoms, so the molecule is not holding all the hydrogen it could.
3. Cit is a gas at room temperature and therefore cannot dissolve any more of itself in water.
4. Dit burns with a smoky yellow flame, which shows that not all of the carbon in the molecule has been used up by the oxygen supplied to the flame.

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1. Aas a fertiliser, because plants take up the carbon in it directly through their leaves.
2. Bas a refrigerant gas, because it evaporates at a very low temperature inside a cooling coil.
3. Cto make polythene, because its molecules join end to end in long chains to form the plastic.
4. Das a solvent for paints and varnishes, because it dissolves oily substances readily.

1. Athe energy given to each unit of charge as it passes through a cell.
2. Bthe opposition that a conductor offers to the flow of charge through it.
3. Cthe total quantity of charge stored on the plates of a capacitor.
4. Dthe rate of flow of electric charge past a point in a circuit.

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1. Aperspex, which is a hard, clear solid and therefore lets charge run over its surface.
2. Bsugar solution, whose dissolved sugar molecules carry the charge through the liquid.
3. Ca carbon rod, in which the graphite structure leaves one electron per atom free to move.
4. Dpure water, whose molecules line up end to end under the applied voltage and pass the charge from one molecule to the next along the chain.

1. Acurrent equals power divided by the square of the voltage.
2. Bcurrent equals power divided by voltage.
3. Ccurrent equals power multiplied by voltage.
4. Dcurrent equals voltage divided by power.

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1. Aabsorbs radiation better than any other surface, so the pot takes in most of the energy falling on it.
2. Bstops the heat inside the pot escaping by convection into the air above the food.
3. Creflects the radiation falling on it back into the curved dish, where the dish concentrates it onto the pot for a second time and so doubles the heating.
4. Dconducts heat along the metal of the pot faster than a white surface of the same thickness.

1. Aparallel rays falling over the whole dish are reflected to meet at one point, where the pot sits.
2. Bdish is stiffer and can be made of much thinner metal than a flat sheet would need.
3. Cdish can be swung round and tilted to follow the sun as it moves across the sky, so that it faces the sun squarely all through the day.
4. Drain that falls on the dish runs off it instead of collecting and rusting the metal.

1. Aa good conductor of heat, so heat travels quickly along the dish towards the pot.
2. Ba good emitter of radiation, so the dish gives out again during the evening all the heat that it has collected from the sun during the day.
3. Ca good reflector of radiation, so nearly all the radiation falling on it is thrown onto the pot.
4. Da good absorber of radiation, so the dish itself becomes hot enough to cook on.

1. Areflection, in which heat bounces off a shiny surface back the way it came, and refraction, in which heat is bent as it passes into a denser material.
2. Bevaporation, in which a liquid turns to vapour at its surface, and condensation, in which the vapour turns back to liquid.
3. Cconduction, in which heat passes from particle to particle, and convection, in which a heated fluid rises and cooler fluid sinks to replace it.
4. Dabsorption, in which a dull surface soaks up heat, and emission, in which a hot surface gives that heat out again.

1. Amuscle cell, which is long and spindle shaped so that it can shorten and pull the bone it is attached to.
2. Bred blood cell, whose biconcave shape and lack of a nucleus leave the maximum room for haemoglobin.
3. Croot hair cell, which has a long narrow extension that pushes between the soil particles to reach water.
4. Dwhite blood cell, which patrols the bloodstream engulfing bacteria and other foreign particles it meets.

1. Ais packed with haemoglobin and shaped as a biconcave disc, giving a large surface area for oxygen to diffuse across quickly.
2. Bhas a large nucleus that stores the instructions for making fresh haemoglobin whenever the supply of it starts to run low.
3. Chas a thick, rigid wall that protects the haemoglobin inside it from the pressure of the blood being pumped by the heart.
4. Dcarries its own supply of digestive enzymes, so that it can break down the oxygen it collects into a form the tissues can use.

1. Alymphocytes make antibodies that clump and destroy pathogens, and phagocytes engulf and digest the invading microbes.
2. Bthe platelets release a clotting protein that traps the pathogens inside the clot until they die of starvation.
3. Cthe plasma carries dissolved food and heat to every organ, which keeps the body strong enough to shrug off any infection it meets.
4. Dthe red blood cells carry oxygen to the site of an infection, and the oxygen kills the bacteria growing there.

1. Alight, oxygen and a suitable temperature, since the seed has to see the light before its enzymes will start to work.
2. Bwater, light and a suitable temperature, since the young shoot must start photosynthesising as soon as it appears.
3. Cwater, carbon dioxide and light, since germination is simply the first stage of photosynthesis in the young seedling.
4. Dwater, oxygen and a suitable temperature, since the enzymes need water to work and respiration needs oxygen.

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1. Asplit open when the flower is ripe so that the pollen grains inside it are shaken out into the wind.
2. Benclose and protect the whole of the flower while it is still tightly closed in the bud.
3. Chold the stigma in position and provide the path down which the pollen tube grows to reach the ovary.
4. Dcarry the sugary nectar up from the base of the flower to the surface of the stigma, where a visiting insect will find it.

1. Aproduce the pollen grains that carry the male sex cells.
2. Bhold the ovules in which the female sex cells are found.
3. Cenclose and protect the flower while it is still a bud.
4. Dattract the insects that will carry the pollen from flower to flower.

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1. Ait produces enormous quantities of very light, smooth, dry pollen, far more than any single visiting insect could possibly carry away.
2. Bits petals are large and brightly coloured, and its anthers and stigma are held inside the flower where a visiting insect must brush past them.
3. Cits petals are small, green and dull to look at, and its stigma is large and feathery and hangs right outside the flower where the moving air can reach it.
4. Dits anthers hang outside the flower on long, slender filaments that swing freely in the slightest breath of moving air.

1. Aovule sends a fine thread up the outside of the style to the stigma and draws the whole pollen grain back down with it.
2. Bpollen grain germinates and grows a pollen tube down through the style and into the ovule, and the male nucleus travels down that tube.
3. Cstigma slowly closes around the grain and carries it down inside itself as the whole length of the style shortens and draws the grain towards the ovary below.
4. Dgrain dissolves into the nectar and is carried down to the ovary by the stream of sugary liquid running back into the flower.

1. Athe growth of hair in the armpits and around the pubic area as the sex hormones begin to circulate.
2. Ban increase in sweating and the appearance of pimples on the face as the skin glands become active.
3. Ca sudden increase in height as the long bones of the body grow rapidly over two or three years.
4. Dthe breaking of the voice as the larynx enlarges and the voice becomes noticeably deeper.

1. Abreakdown of glucose in living cells without oxygen, releasing a small amount of energy and producing lactic acid.
2. Bbreakdown of glucose in living cells using oxygen, releasing energy and producing carbon dioxide and water.
3. Cbuilding up of glucose from carbon dioxide and water in the chloroplasts, using energy taken from sunlight.
4. Dbreathing movements of the chest that draw air into the lungs and then push the used air out again.

1. Amore oxygen and more carbon dioxide, because both of those gases are given out by the respiring cells of the body tissues and collected by the blood.
2. Bmore oxygen and less carbon dioxide, because the lungs manufacture oxygen out of the water vapour in the blood.
3. Cless oxygen and more carbon dioxide, because the tissues use oxygen in respiration and give out carbon dioxide.
4. Dthe same amount of oxygen and of carbon dioxide as before, because the two gases simply pass straight through the lungs unchanged on their way out again.

1. Athe mucus of the nose adds heat to the air by the chemical reaction that produces it.
2. Bit has been warmed to body temperature and saturated with water vapour while it was inside the lungs.
3. Cthe moving air is heated by the friction of rushing along the narrow tubes of the windpipe and the bronchi on its way back out of the chest.
4. Dwater is manufactured from the oxygen in the air by the cells that line the inside of the trachea.

1. Acarbon dioxide and water, which are the same two substances a burning fuel gives off.
2. Boxygen and glucose, which are the substances a green leaf makes when the sun is shining on it.
3. Clactic acid and carbon dioxide, which build up in the cell until the oxygen supply is restored.
4. Dethanol and oxygen, which the plant releases through the open stomata of its leaves.

1. Ahairs physically block the pore so that no water vapour at all can escape from the leaf into the air outside it.
2. Bpit collects rain water and holds it against the pore, so that the leaf is able to take some of that water back in through the stoma when it needs it.
3. Chairs reflect the sunlight away from the pore, so the guard cells never become warm enough to open at all.
4. Dpit traps a pocket of humid air over the pore, so the diffusion gradient out of the leaf is small and water vapour leaves slowly.

1. Athe spines make the surface exposed to the air very small and the waxy layer is waterproof, so water can leave only through the stomata.
2. Bthe spines catch the wind and cool the plant down, while the waxy layer traps a layer of warm air against the surface of the leaf.
3. Cthe spines store all the water the plant will need to carry it through the dry season, and the waxy layer stops any thirsty animal biting into that store.
4. Dthe spines carry out the whole of the plant's photosynthesis, and the waxy layer reflects enough sunlight to keep the plant cool.

1. Aspeed up the reaction between the liquid and any metal carbonate that is added to the test tube after the test.
2. Bshow whether the liquid is acidic, neutral or alkaline, and roughly how strongly so, by the colour it turns.
3. Cneutralise any acid that is present in the liquid so that the tube can be handled safely.
4. Dmeasure the exact volume of acid that would be needed to neutralise the liquid in the tube completely.

1. AB is neutral and C is alkaline, since green is the colour at pH 7 and purple is the colour at the alkaline end.
2. BB is alkaline and C is neutral, since green shows the presence of hydroxide ions and purple shows a balance of ions.
3. CB is a weak acid and C is a weak alkali, since green and purple both lie close to the middle of the pH scale.
4. DB is a strong acid and C is a strong alkali, since green sits at the very bottom of the pH scale and purple at the top.

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1. Amagnesium chloride, water and carbon dioxide, since an acid reacting with a carbonate always gives all three of those.
2. Bmagnesium hydroxide and carbonic acid, since the two compounds simply exchange their metal and non metal parts.
3. Cmagnesium chloride and hydrogen only, since an acid reacting with any magnesium compound at all will set hydrogen free.
4. Dmagnesium oxide, chlorine and water, since the acid breaks up and its chlorine is released as a choking green gas.

1. Acomplete removal of all oxygen from a container, leaving a vacuum inside it.
2. Bloss of oxygen by a substance, which is what happens to a metal ore when it is smelted in a furnace.
3. Ctransfer of oxygen from a solid to a gas, which is what happens whenever a fuel is burnt.
4. Dgain of oxygen by a substance, which is what happens to magnesium when it burns in air.

1. Again of electrons by an atom or ion, which makes the particle more negatively charged.
2. Bsharing of a pair of electrons between two atoms, which forms a covalent bond between them.
3. Closs of electrons by an atom or ion, which makes the particle more positively charged.
4. Dmovement of electrons round the nucleus from an inner shell to an outer shell of the atom.

1. Aboth FeCl2 and CuO are reduced, because in each reaction another substance is added to them by the reaction.
2. BFeCl2 is oxidised because its iron loses an electron, and CuO is reduced because it loses its oxygen.
3. CFeCl2 is reduced because its iron gains an electron, and CuO is oxidised because it gains hydrogen from the gas.
4. Dboth FeCl2 and CuO are oxidised, because in each reaction they are the substance that is chemically changed.

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1. Athe molten slag, which is light enough to be carried up the furnace as a vapour.
2. Bchlorine and sulphur dioxide, driven off from the limestone as it is heated.
3. Cpure oxygen, left over from the hot air blast that was forced in at the base.
4. Dthe waste gases, mostly nitrogen with carbon dioxide and some carbon monoxide.

1. Aoxidised by the hot air blast, which drives the oxygen out of the ore and into the waste gases.
2. Bheated on its own to a high enough temperature for it to break apart into iron and oxygen.
3. Creduced by carbon monoxide, which takes the oxygen away and leaves the iron behind.
4. Ddissolved in the molten slag, from which the iron then settles out because it is denser.

1. Areacts with the iron to harden it, which is what turns the soft molten iron into a usable steel.
2. Bdecomposes to calcium oxide, which combines with the sandy impurity in the ore to form the slag.
3. Cdissolves in the molten iron and lowers its melting point so that it can be run off more easily.
4. Dburns fiercely in the hot air blast and so supplies most of the heat the furnace needs to melt the ore.

1. Acools the iron quickly, so the metal can be tapped off and cast within a few minutes.
2. Bdissolves the carbon out of the iron, which is what makes the iron soft enough to shape.
3. Cseals the hot iron off from the air, so the iron is not oxidised back into its oxide.
4. Dadds calcium to the iron, which is what gives the finished metal its resistance to rusting.

1. A2 to 3, since 70 divided by 56 gives 1.25 and 30 divided by 16 gives 1.875.
2. B3 to 2, since the mass of iron is a little more than twice the mass of oxygen present.
3. C1 to 1, since the two elements are always combined in equal numbers of atoms in an oxide.
4. D3 to 4, since the two figures 1.25 and 1.875 are in the same ratio as 3 is to 4.

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1. Apassing an electric current through molten sulphur until the gas bubbles off at the anode.
2. Bburning sulphur in air or oxygen, or roasting a sulphide ore such as zinc blende in air.
3. Cadding dilute sulphuric acid to a metal carbonate and collecting the gas that fizzes off.
4. Dheating sulphur trioxide with a catalyst so that it breaks down into the dioxide and oxygen.

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1. Aa temperature of about 1500 degrees Celsius and a pressure of about 200 atmospheres, as in the blast furnace.
2. Ba temperature of about 450 degrees Celsius and a pressure of only about 1 to 2 atmospheres.
3. Ca temperature of about 25 degrees Celsius and a pressure of about 50 atmospheres, so the gases stay liquid.
4. Da temperature of about 450 degrees Celsius and a pressure of about 250 atmospheres, as in the Haber process.

1. Ais so violent that it throws off a dense acid mist that is dangerous and very hard to condense.
2. Bwould produce a compound that is far weaker than sulphuric acid and of no industrial use at all.
3. Cwould cool the mixture so sharply that the plant would have to be heated again from the start.
4. Dcannot happen at all, because sulphur trioxide is completely insoluble in cold or warm water.

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1. ASO3 + H2O giving H2SO4, because the trioxide is what actually reacts with the added water.
2. BH2S2O7 + H2O giving 2H2SO4, because the oleum takes up the water and splits into two acid molecules.
3. CH2SO4 + H2O giving H2SO4 + H2O, because diluting an acid is only a physical change and not a chemical reaction at all.
4. D2H2SO4 giving H2S2O7 + H2O, because the acid loses water to form the oleum in this stage.

1. Afor softening hard water in boilers, because it removes the calcium ions that form the scale.
2. Bas an antiseptic in hospitals, because its low pH kills the bacteria on a wound or an instrument.
3. Cin the manufacture of fertilisers such as ammonium sulphate and superphosphate for farming.
4. Das a fuel in industrial furnaces, because a great deal of heat is released when it is burnt in air.

1. Astays at rest, or keeps moving in a straight line at a constant speed, unless a resultant external force acts on it.
2. Baccelerates in the direction of the resultant force acting on it, at a rate proportional to that force and inversely proportional to its own mass.
3. Cexerts on every other body a force that is equal in size and opposite in direction to the force that the other body is exerting on it.
4. Dfalls towards the earth with an acceleration that does not depend at all on how much mass the falling body happens to have.

1. Aacceleration equals force divided by the square of the mass.
2. Bacceleration equals force divided by mass.
3. Cacceleration equals force multiplied by mass.
4. Dacceleration equals mass divided by force.

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1. Athe inlet valve is open and the rising piston squeezes the mixture of petrol and air into a very small space.
2. Bboth valves are open and the piston is drawn down the cylinder, pulling in a fresh charge of petrol and air.
3. Cthe exhaust valve is open and the rising piston pushes the burnt gases out of the cylinder and into the exhaust pipe behind it.
4. Dboth valves are closed, the spark plug fires and the burning gases expand and drive the piston down the cylinder.

1. Athe induction stroke only, when the fresh mixture is being drawn into the cylinder by the falling piston.
2. Bthe compression stroke and the power stroke, which are told apart by the direction the piston is moving.
3. Cthe exhaust stroke only, when the burnt gases are being pushed out of the cylinder by the rising piston.
4. Dall four of the strokes, since the valves are opened only while the engine is being started from cold.

1. Aruns without any oil in it, because its moving parts are made of a material that has no friction at all.
2. Bgives out no exhaust gases whatever, because everything the engine burns is turned into useful work at the crankshaft.
3. Cinjects a measured spray of fuel instead of drawing it through a carburettor, so it burns fuel more completely.
4. Dneeds no fuel at all once it has been started, because it runs on the heat it has already produced.

1. Aprinting of information onto sheets of paper so that many copies of it can be distributed to different readers.
2. Bconversion of sound energy into electrical energy by a microphone placed in front of the speaker.
3. Csending and receiving of information over a distance using electrical signals or electromagnetic waves.
4. Dstoring of information on a magnetic tape or a disc so that it can be read back at some later time.

1. AA printing press, which produces thousands of copies of a newspaper for readers to buy in the morning.
2. BA public address system, which makes one speaker's voice loud enough to be heard across a school assembly.
3. CA filing cabinet, which stores letters and records so that a clerk can retrieve any one of them later.
4. DA cellphone network, which carries a caller's speech through base stations to a handset far away.

1. Aamplifier and the aerial, which are the two parts that make the signal strong enough to be heard.
2. Bmodulator and the demodulator, which are the two parts that put the message onto the carrier wave.
3. Ctransmitter and the receiver, which send the signal into the medium and pick it up at the far end.
4. Dmicrophone and the loudspeaker, which are the two parts that handle sound at either end of the link.

1. Astore the message until the receiver at the far end of the link is ready to accept it and asks for it to be sent.
2. Bpick the signal out of the medium, separate the message from the carrier wave and turn it back into sound or a picture.
3. Ccarry the signal from one place to another, either as a current in a cable or as a radio wave travelling through space.
4. Dconvert the message into a signal, modulate it onto a carrier wave and feed the amplified result into the medium.

1. Ais completely free to use, because a call carried by radio waves costs the network nothing at all to connect.
2. Bgives a clearer sound on every call, because a radio signal can never be weakened or interrupted at all.
3. Cneeds no battery, because it draws all the power it uses from the radio waves coming from the base station.
4. Dcan be carried about, so the user can make and receive calls anywhere within the coverage of the network.

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1. Acoal will be replaced naturally within a few years, so a station must be shut down while the seams reform.
2. Bcoal cannot be transported by road or rail, so a station has to be built directly on top of the coal seam.
3. Ccoal releases so little heat when it burns that a very large furnace is needed to raise any steam at all.
4. Dburning coal gives off carbon dioxide and sulphur dioxide, which cause global warming and acid rain.

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1. Ais condensed straight back into water, so that the very same water can be pumped round and used to raise steam again.
2. Bis compressed into a smaller volume so that its pressure rises high enough to drive the generator.
3. Cis heated further until it is hot enough to be turned directly into an electric current by the blades.
4. Dpushes against the curved blades and spins the shaft, so the energy of the steam becomes kinetic energy.

1. Araising the voltage of an electric current that is already flowing, so that less heat is lost in the cables.
2. Bseparating the positive from the negative charges in a chemical solution, in exactly the way that a dry cell does.
3. Cturning a coil in a magnetic field, so that a voltage is induced across the coil and a current flows.
4. Dstoring the charge produced by the turning shaft on two metal plates until enough of it has built up to be useful.

1. Aelectrical energy to kinetic energy, then kinetic energy to potential energy of the stored water above.
2. Bpotential energy to kinetic energy, then kinetic energy to electrical energy.
3. Cnuclear energy to heat energy, then heat energy to kinetic energy, then kinetic energy to electrical energy.
4. Dchemical energy to heat energy, then heat energy to kinetic energy, then kinetic energy to electrical energy.