Why did I lose the mark?

The answer that scoresA species is a group of organisms that can reproduce to produce fertile offspring. The offspring must be fertile: a horse and a donkey can breed, but the mule they produce is sterile, so horses and donkeys are separate species.

The answer that scoresThe scientific name is Homo sapiens: genus Homo, with a capital H, followed by species sapiens, with a lower case s, with the whole name in italics or underlined. The genus always comes first and the species second.

The answer that scoresViruses are not placed in any kingdom because they are not cells. A virus consists of genetic material surrounded by a protein coat, and it can only reproduce inside a living host cell, so it cannot carry out the life processes on its own.

The answer that scoresEach couplet must offer two opposite statements about the same observable feature, for example '1a has wings ... go to 2' and '1b has no wings ... go to 3'. Every specimen must fit exactly one statement at each step, so it reaches one name only.

The answer that scoresAll cells have a cell membrane, a thin partially permeable layer that controls what enters and leaves the cell. A plant cell also has a cell wall, made of cellulose, outside the cell membrane. The cell wall strengthens the cell and gives it a fixed shape, but it is fully permeable, so it does not control what enters the cell - that is the membrane's job.

The answer that scoresA bacterial cell has no nucleus, so its DNA is not enclosed in a membrane. It carries one loop of circular DNA free in the cytoplasm, which holds the genes that control the cell's activities, and it may also contain plasmids - small rings of extra DNA that often carry genes such as those for antibiotic resistance.

The answer that scoresA mature red blood cell has no nucleus, which leaves more space for haemoglobin, so each cell can carry more oxygen. It is also shaped like a biconcave disc, which gives it a large surface area for oxygen to diffuse in and out quickly. The trade-off is that it cannot divide or repair itself, so it survives only about four months before being replaced.

The answer that scoresmagnification = image size / actual size. Rearranged, actual size = image size / magnification and image size = actual size x magnification. If a cell measures 30 mm in an image magnified 1500 times, the actual size is 30 mm / 1500 = 0.02 mm, which is 20 micrometres.

The answer that scoresOsmosis is the net movement of water molecules from a region of higher water potential (a dilute solution) to a region of lower water potential (a more concentrated solution), through a partially permeable membrane. Only water moves, and it moves towards the more concentrated solution.

The answer that scoresActive transport moves particles through a cell membrane from a region of lower concentration to a region of higher concentration - against the concentration gradient - using energy from respiration and protein carriers in the membrane.

The answer that scoresWater leaves the cell by osmosis because the solution outside has a lower water potential than the cell contents. The cell contents shrink and stop pressing outwards on the cell wall, so the cell becomes flaccid. If enough water is lost, the cytoplasm and cell membrane pull away from the cell wall and the cell is plasmolysed.

The answer that scoresThe energy for diffusion comes from the kinetic energy of the random movement of the molecules and ions themselves. No energy from respiration is needed, so diffusion is a passive process and continues even in a dead cell. Energy from respiration is needed only for active transport, which moves particles against a concentration gradient.

The answer that scoresProteins contain carbon, hydrogen, oxygen and nitrogen. Carbohydrates and fats contain carbon, hydrogen and oxygen only, so nitrogen is the element that identifies a molecule as a protein.

The answer that scoresStarch and cellulose are both polymers of glucose, but the glucose units are joined in different arrangements. Human digestive enzymes can hydrolyse the bonds in starch but not the bonds in cellulose, so cellulose passes through the gut undigested as dietary fibre, adding bulk and helping to prevent constipation.

The answer that scoresBenedict's solution is added to the sample and the mixture is heated, for example in a water bath. A positive result is a colour change from blue to a brick-red precipitate, and the colour passes through green, yellow and orange as the concentration of reducing sugar increases.

The answer that scoresThe biuret test is carried out at room temperature: biuret solution is added to the sample and a positive result for protein is a colour change from blue to purple (violet). No heating is required.

The answer that scoresAt high temperature the bonds that hold the enzyme's folded shape break. The active site changes shape, so the substrate no longer fits it and no enzyme-substrate complexes can form. The enzyme is denatured, which is a permanent change - cooling it down does not restore the shape.

The answer that scoresThe enzyme is not changed by the reaction. The substrate binds to the active site, is converted into products, and the products are released, leaving the enzyme exactly as it was. The same enzyme molecule can therefore catalyse the reaction again and again, which is why only a small amount is needed.

The answer that scoresEach enzyme has an active site with a shape complementary to one particular substrate. A molecule with a different shape does not fit the active site, so no enzyme-substrate complex forms and no reaction is catalysed. Amylase digests starch but has no effect on protein because protein does not fit its active site.

The answer that scoresAt low temperature the enzyme and substrate molecules have less kinetic energy, so they move more slowly, collide less often and collide with less energy, so there are fewer effective collisions and the rate is low. The enzyme is not denatured, so warming it restores the rate.

The answer that scoresPhotosynthesis takes in carbon dioxide and water and uses energy from light, trapped by chlorophyll, to make glucose and oxygen: carbon dioxide + water -> glucose + oxygen. Glucose is the carbohydrate product and oxygen is released as a by-product.

The answer that scoresAfter destarching and testing with iodine, only the green parts of a variegated leaf turn blue-black. The white patches contain no chlorophyll, so they cannot absorb light and cannot make glucose or starch. This is the evidence that chlorophyll is needed for photosynthesis.

The answer that scoresAs light intensity increases the rate of photosynthesis rises at first, because more light is absorbed by chlorophyll. Above a certain intensity the rate levels off (plateaus), because light is no longer the limiting factor - carbon dioxide concentration or temperature has become limiting instead.

The answer that scoresNitrate ions are needed for making amino acids, and therefore proteins, so a shortage stunts growth. Magnesium ions are needed for making chlorophyll, so a shortage turns the leaves yellow because the plant cannot make enough chlorophyll to absorb light.

The answer that scoresChewing is physical digestion: it breaks food into smaller pieces without any chemical change to the molecules, and its purpose is to increase the surface area so that enzymes can act faster. Chemical digestion is different - it uses enzymes to break large insoluble molecules into small soluble molecules that can be absorbed, for example amylase breaking starch down to maltose.

The answer that scoresThe pancreas produces pancreatic juice, which contains amylase, protease and lipase, and releases it into the duodenum. The liver produces bile, which is stored in the gall bladder and released into the duodenum. Bile contains no enzymes: it is alkaline, so it neutralises the acid from the stomach, and it emulsifies fats, breaking large droplets into smaller ones to increase the surface area for lipase.

The answer that scoresGlucose and amino acids are absorbed into the dense network of capillaries in the villus and are carried away in the blood, which also keeps the concentration gradient steep. The lacteal is the vessel at the centre of the villus and it absorbs the fatty acids and glycerol from fat digestion, which are reassembled into fats before they eventually enter the blood.

The answer that scoresScurvy is caused by too little vitamin C. Vitamin C is needed to make collagen, so without it the gums bleed and swell, teeth loosen and wounds heal slowly. Rickets is caused by too little vitamin D, or too little calcium: vitamin D is needed for calcium to be absorbed from the intestine and deposited in bone, so without it the bones stay soft and bend, and the leg bones bow.

The answer that scoresXylem transports water and mineral ions from the roots to the leaves, and also supports the plant. Phloem transports sucrose and amino acids from sources to sinks, in both directions, because these substances are loaded into the phloem wherever they are produced and removed wherever they are needed.

The answer that scoresTranspiration is the loss of water vapour from the leaves. Water evaporates from the surfaces of the mesophyll cells into the air spaces and then diffuses out of the leaf through the stomata. The movement of water up the xylem to replace it is called the transpiration stream.

The answer that scoresTranspiration is slower when the air is more humid. A high humidity means the water vapour concentration outside the leaf is closer to the concentration inside the leaf, so the diffusion gradient between the air spaces and the outside air is shallower and water vapour diffuses out more slowly.

The answer that scoresXylem vessels are dead at maturity. Their thick walls contain lignin, which makes them strong and waterproof; they have no cell contents, so the tube is hollow; and their cells are joined end to end with no cross walls, so they form one long continuous tube through which water can flow freely from the roots to the leaves.

The answer that scoresArteries carry blood away from the heart and veins carry blood back to the heart. Most arteries carry oxygenated blood, but the pulmonary artery carries deoxygenated blood from the heart to the lungs, and the pulmonary vein carries oxygenated blood from the lungs back to the heart.

The answer that scoresThe pulmonary artery carries deoxygenated blood away from the heart (from the right ventricle) to the lungs. The pulmonary vein carries oxygenated blood from the lungs back to the heart, into the left atrium.

The answer that scoresRed blood cells have no nucleus, which leaves more room for haemoglobin inside the cell. The haemoglobin combines with oxygen in the lungs to form oxyhaemoglobin and releases it in the tissues. Their biconcave shape gives a large surface area to volume ratio, so oxygen diffuses in and out quickly.

The answer that scoresThe left ventricle has the thicker muscular wall because it pumps blood all round the body, so it must generate a high pressure. The right ventricle pumps blood only to the lungs, a much shorter journey, so it needs only a low pressure and has a thinner wall.

The answer that scoresThe skin is a mechanical barrier. Its tough outer layer is dead and dry, so pathogens cannot pass through it, and any cut is quickly sealed by a blood clot. The chemical barriers are substances such as the hydrochloric acid in the stomach and the enzymes in mucus, which kill pathogens rather than simply blocking them.

The answer that scoresA vaccine contains weakened, dead or inactive pathogens, or their antigens. These antigens stimulate an immune response by lymphocytes, which produce antibodies, and memory cells are produced that give long-term immunity. Because the body makes its own antibodies, this is active immunity. A vaccine that supplied ready-made antibodies would give only passive, short-term immunity.

The answer that scoresThe mosquito is the vector. The pathogen that causes malaria is the protoctist Plasmodium, which the mosquito picks up when it bites an infected person and injects into the next person it bites. The mosquito transmits the pathogen but does not cause the disease itself.

The answer that scoresBreast milk passes ready-made antibodies from the mother to the baby, which is passive immunity. No memory cells are produced, and the borrowed antibodies are gradually broken down, so the protection lasts only a few months. Active immunity, gained by infection or vaccination, is the type that produces memory cells and lasts much longer.

The answer that scoresVentilation (breathing) is the movement of air into and out of the lungs, caused by the ribs, intercostal muscles and diaphragm. Respiration is the chemical reactions in cells that break down nutrient molecules such as glucose and release energy for metabolism. Ventilation supplies the oxygen that aerobic respiration needs and removes the carbon dioxide it produces, but it releases no energy itself.

The answer that scoresNitrogen makes up about 78% of both inhaled and exhaled air. The body does not use nitrogen in respiration, so it is breathed in and out unchanged. The gases that change are oxygen (falls from about 21% to about 16%), carbon dioxide (rises from about 0.04% to about 4%) and water vapour (increases).

The answer that scoresDuring inhalation the diaphragm contracts, flattens and moves down, and the external intercostal muscles contract so the ribs move up and out. The volume of the thorax increases and the pressure inside falls below atmospheric pressure, so air is drawn in. During exhalation the diaphragm relaxes and becomes domed, moving up, and the ribs move down and in, so the volume decreases and air is pushed out.

The answer that scoresCarbon monoxide combines with haemoglobin in red blood cells more readily than oxygen does, reducing the amount of oxygen the blood can carry, so less oxygen reaches the tissues. Lung cancer is caused by the carcinogens in tar, which cause mutations in the cells lining the airways and lead to uncontrolled cell division. Nicotine is the addictive stimulant, and emphysema is caused by the destruction of the alveolar walls.

The answer that scoresBreathing is the movement of air into and out of the lungs, brought about by the ribs, diaphragm and intercostal muscles. Respiration is the chemical reactions in cells that break down nutrient molecules such as glucose and release energy for metabolism. Breathing supplies the oxygen for aerobic respiration and removes the carbon dioxide that respiration produces.

The answer that scoresIn yeast, anaerobic respiration produces alcohol (ethanol) and carbon dioxide: glucose -> alcohol + carbon dioxide. In muscles during vigorous exercise it produces lactic acid: glucose -> lactic acid. The two reactions start from the same substrate but end with different products.

The answer that scoresAnaerobic respiration releases much less energy per glucose molecule than aerobic respiration, because the nutrient molecule is not broken down completely - glucose is converted only to lactic acid in muscle, or to alcohol and carbon dioxide in yeast. Aerobic respiration breaks glucose down completely to carbon dioxide and water, so it releases a relatively large amount of energy.

The answer that scoresLactic acid is carried in the blood from the muscles to the liver. The heart rate stays fast to transport it, and breathing stays deep and fast to supply the oxygen. In the liver the lactic acid is respired aerobically, using that oxygen, and converted back into glucose. The debt is repaid when the lactic acid has been removed - not by breathing it out.

The answer that scoresA reflex action is rapid and automatic because the impulse follows a short pathway: receptor -> sensory neurone -> relay neurone in the spinal cord -> motor neurone -> effector. The brain is not involved, so no conscious thought is needed and the response is very fast. The brain is informed afterwards, which is why you feel the pain after your hand has already moved.

The answer that scoresThe iris controls how much light enters the eye by changing the diameter of the pupil, using the antagonistic circular and radial muscles. In bright light the circular muscles contract and the pupil constricts; in dim light the radial muscles contract and the pupil dilates. The lens does a different job: it changes shape to focus light onto the retina, which is accommodation.

The answer that scoresInsulin decreases the blood glucose concentration: it causes the liver to convert glucose into glycogen for storage and increases the uptake of glucose by cells. Glucagon increases the blood glucose concentration: it makes the liver convert stored glycogen back into glucose. They are antagonistic hormones and together they keep blood glucose close to the set point.

The answer that scoresLight causes auxin to move from the lit side to the shaded side of the shoot. The shaded side therefore has more auxin, its cells elongate more, and it grows longer than the lit side, so the shoot bends towards the light. That is positive phototropism, and it gives the leaves the best possible access to light for photosynthesis.

The answer that scoresAntibiotics kill bacteria but have no effect on viruses. Influenza is caused by a virus, which has no bacterial cell wall or metabolism for the antibiotic to attack, so an antibiotic cannot cure flu. The patient needs rest and symptom relief while the immune system clears the virus.

The answer that scoresThe bacteria became resistant. A random mutation gave some bacteria resistance to the antibiotic; when the antibiotic was used it killed the non-resistant bacteria, and the resistant survivors reproduced, so the bacterial population is now mostly resistant and the antibiotic is less effective.

The answer that scoresAlcohol is a depressant: it slows the transmission of nervous impulses at the synapse, which impairs judgement, slows reaction time and reduces coordination. It may make a person feel more confident, but that is a social effect, not evidence that it speeds up the nervous system.

The answer that scoresThe patient must complete the full course. Stopping when the symptoms improve leaves some bacteria alive; these multiply, and if any are less affected by the antibiotic, resistance becomes more likely and the infection can return.

The answer that scoresPollination is the transfer of pollen grains from an anther to a stigma. Fertilisation happens afterwards: a pollen tube grows down the style to the ovule, and a pollen nucleus fuses with a nucleus in the ovule. Pollination is a transfer; fertilisation is a fusion of nuclei.

The answer that scoresThe mother's blood and the fetus's blood do not mix. They are separated by the placenta, and substances move across it by diffusion: dissolved nutrients such as glucose, oxygen and antibodies pass from the mother to the fetus, while carbon dioxide and other excretory products pass from the fetus to the mother. The umbilical cord carries the fetus's blood to and from the placenta.

The answer that scoresGermination requires water, oxygen and a suitable temperature - not light, and not soil. Water softens the seed coat and activates the enzymes that break down the food store; oxygen is needed for aerobic respiration to release energy; and a suitable temperature keeps those enzymes working at the right rate. The seedling uses the food store inside the seed, not nutrients from the soil.

The answer that scoresAfter fertilisation the ovule becomes the seed, because the ovule contains the fertilised egg cell, and the ovary becomes the fruit, which develops around the seeds to protect them and help in dispersal.

The answer that scoresThe genotypic ratio is 1 TT : 2 Tt : 1 tt, so only one plant in four is homozygous dominant and two in four are heterozygous. Because TT and Tt both show the dominant feature, the phenotypic ratio is 3 tall : 1 short. Always give the genotype ratio and the phenotype ratio separately.

The answer that scoresA gamete contains one allele of each gene, because it has a single set of chromosomes (23 in humans). A diploid body cell contains two alleles of each gene, one on each chromosome of a pair. This is why the two alleles of a gene separate when gametes are formed.

The answer that scoresIn codominance both alleles in a heterozygous organism contribute to the phenotype, so both are seen at once - for example a person with the genotype I^A I^B has both the A and the B antigens on their red blood cells and has blood group AB. Neither allele is hidden and neither is diluted.

The answer that scoresSickle cell anaemia is caused by a mutation that changes the base sequence of the haemoglobin gene, so the abnormal haemoglobin HbS is made and red blood cells become sickle-shaped. Carriers (HbA HbS) are healthy and have some resistance to malaria, because the malarial parasite develops less well in their red blood cells, but they are not immune to it.

The answer that scoresThe mutation that gives resistance arises at random, so a few bacteria in the population are already resistant before the antibiotic is taken. When the antibiotic is used, it kills the non-resistant bacteria, so the resistant bacteria survive, reproduce and pass on the resistance allele. Over many generations the resistant strain becomes common, and the antibiotic no longer works.

The answer that scoresHeight shows continuous variation because it results in a range of phenotypes between two extremes, with no intermediates or separate categories, and it is measured on a scale. It is caused by both genes and the environment and is plotted as a normal distribution curve. Discontinuous variation, such as ABO blood group, has a limited number of distinct phenotypes with no intermediates and is caused by genes only.

The answer that scoresSelective breeding can raise yield, improve quality and give resistance to disease, so it benefits the farmer and the food supply. However, breeding from a small number of individuals reduces genetic variation and can cause inbreeding, so the population may become more susceptible to a new disease, because there may be no resistant individuals left. Genetic engineering is an alternative that introduces a single desired gene more quickly and precisely.

The answer that scoresAdaptation is the process, resulting from natural selection, by which populations become more suited to their environment over many generations. Individuals that happen to have advantageous inherited features are more likely to survive and reproduce, so they pass those alleles on and the advantageous alleles become more common in the population. The individual does not change; the population does.

The answer that scoresA population is a group of organisms of one species, living in the same area, at the same time. A community is all of the populations of different species in an ecosystem. So a pond may contain a population of water lilies and a population of frogs, and together all its populations make up the community.

The answer that scoresOnly about 10 per cent of the energy is transferred to the next trophic level because most of it is lost: as heat released in respiration, in movement, in excretory products such as urea, and in parts of the organism that are not eaten, such as bones, roots and fur. Very little is stored as growth.

The answer that scoresThe arrow shows the direction of energy transfer, so it points from the organism being eaten to the organism that eats it: grass -> rabbit -> fox. Energy passes from the grass into the rabbit when the rabbit feeds, and then into the fox when the fox eats the rabbit.

The answer that scoresA pyramid of numbers can be inverted, because a single large producer may support a very large number of small consumers - one oak tree and the insects that feed on it. A pyramid of biomass is better because it compares the mass of living material and takes account of the size of the organisms, and a pyramid of energy is better still because it measures the energy available at each level and is never inverted.

The answer that scoresNitrate ions leached from the fields increase the availability of nutrients, so the growth of producers such as algae increases. When the producers die, the decomposers that break them down increase in number and respire aerobically, which reduces the dissolved oxygen in the water. Fish and other organisms that require dissolved oxygen then die.

The answer that scoresSulphur dioxide, released by burning fossil fuels that contain sulphur, dissolves in rainwater to form acid rain, which damages lakes, soil, trees and buildings. Carbon dioxide is a greenhouse gas; the extra carbon dioxide released by burning fossil fuels and by deforestation increases the greenhouse effect, trapping more heat and causing global warming.

The answer that scoresA monoculture has advantages of efficiency - machinery can be used over the whole field and planting and harvesting are cheaper - but it reduces biodiversity, because only one plant species is grown, and a single pest or disease can destroy the entire crop. It also usually needs heavy applications of fertiliser and pesticide.

The answer that scoresDeforestation reduces biodiversity because the species living in the forest lose their habitat, food and breeding sites, and species found only there may become extinct. The soil is lost because there are no roots to hold it, flooding increases because rainwater runs off the bare ground, and carbon dioxide in the atmosphere rises because the trees that removed it by photosynthesis have gone and burning the timber releases more.

The answer that scoresWhen a gas is heated the particles gain kinetic energy and move faster, so they collide with the walls of the container harder and more often and push them further apart. The volume of the gas increases because the particles are further apart, not because the particles themselves get bigger.

The answer that scoresThe temperature stays constant during boiling. The energy supplied is used to overcome the forces between the particles, so the particles gain potential energy rather than kinetic energy, and the thermometer reading does not change until all the liquid has boiled.

The answer that scoresBoiling happens throughout the liquid, only at the boiling point, and needs a continuous supply of energy. Evaporation happens only at the surface of the liquid, at any temperature, and is faster when the temperature is higher, the surface area is larger or air is moving over the surface.

The answer that scoresThe ring forms nearer the hydrochloric acid end. Ammonia has a lower relative molecular mass, so its molecules move faster and diffuse further in the same time, meeting the slower hydrogen chloride molecules closer to the hydrochloric acid end of the tube.

The answer that scoresIn solid sodium chloride the ions are held in fixed positions in the giant lattice, so they cannot move and cannot carry charge, and the solid does not conduct. When the compound is melted or dissolved in water the lattice breaks down, the ions become free to move, and the molten or aqueous compound conducts well.

The answer that scoresIn a compound the different elements are chemically combined in fixed proportions and can be separated only by a chemical reaction, and the elements lose their own properties. In a mixture the substances are not chemically combined, the proportions can be varied, each substance keeps its own properties, and the mixture can be separated by a physical method such as filtration or distillation.

The answer that scoresIsotopes of an element have the same number of protons and the same number of electrons, so they have the same electronic configuration and therefore the same chemical properties. The different number of neutrons changes the mass, so physical properties such as density differ.

The answer that scoresWhen a simple molecular compound boils, the molecules stay intact and move further apart. Only the weak intermolecular forces between the molecules have to be overcome, and that needs very little energy, so the boiling point is low. The strong covalent bonds inside each molecule are not broken.

The answer that scoresBalance by changing the large number in front of a formula instead. For the combustion of hydrogen, 2H2 + O2 -> 2H2O is balanced: there are four hydrogen atoms and two oxygen atoms on each side, and the substances are unchanged.

The answer that scoresOxygen gas is made of diatomic molecules, O2, so Mr = 2 x 16 = 32 and one mole of oxygen gas has a mass of 32 g. The same applies to hydrogen, H2, nitrogen, N2 and chlorine, Cl2.

The answer that scoresThe volume is 0.50 x 24 = 12 dm3, which is 12000 cm3. Always write the unit that matches the quantity you calculated, and check whether the question asks for dm3 or cm3.

The answer that scoresThe mole ratio Mg : MgO is 2 : 2, which is 1 : 1, so 0.20 mol of magnesium gives 0.20 mol of magnesium oxide. Mr of MgO = 24 + 16 = 40, so mass = 0.20 x 40 = 8.0 g.

The answer that scoresIn electrolysis the anode is the positive electrode and the cathode is the negative electrode. Positive ions (cations) are attracted to the negative cathode, where they gain electrons and are reduced, and negative ions (anions) are attracted to the positive anode, where they lose electrons and are oxidised.

The answer that scoresThe spoon is made the cathode, the negative electrode, because positive silver ions are attracted to it and are reduced to silver metal on its surface. The anode is made of pure silver, and the electrolyte is a solution of a silver salt such as silver nitrate.

The answer that scoresIn dilute sodium chloride solution there are only a few chloride ions, so hydroxide ions from the water are oxidised at the anode instead: 4OH- -> O2 + 2H2O + 4e-. The product at the anode is oxygen, which relights a glowing splint. Chlorine is produced at the anode only from concentrated sodium chloride solution, where the concentration of chloride ions is high.

The answer that scoresA hydrogen-oxygen fuel cell uses hydrogen and oxygen to produce electricity, and the only chemical product is water: 2H2 + O2 -> 2H2O. No carbon dioxide, carbon monoxide or oxides of nitrogen are released at the point of use, which is why it does not pollute the air where it is used.

The answer that scoresIn an exothermic reaction thermal energy is transferred from the reaction mixture to the surroundings. The mixture loses energy and the surroundings gain it, so the temperature of the surroundings increases. The enthalpy change, delta H, is negative.

The answer that scoresBond breaking is endothermic, because energy must be transferred to the molecule to pull the atoms apart. Bond making is exothermic, because energy is transferred to the surroundings as the new bond forms. The enthalpy change of a reaction is the balance between the two.

The answer that scoresCombustion is exothermic. A flame supplies the activation energy, the minimum energy that colliding particles must have in order to react. Once the reaction starts, more energy is released in bond making than is absorbed in bond breaking, so delta H is negative and the surroundings get hotter.

The answer that scoresActivation energy is the minimum energy that colliding particles must have in order to react, and it is never zero. Even in an exothermic reaction the bonds in the reactants must be broken first, which needs energy, so the curve rises to a peak before falling to the lower energy of the products.

The answer that scoresA catalyst increases the rate of reaction by providing an alternative pathway with a lower activation energy, and it is not changed or used up. The same catalyst particles can be used again and again, which is why only a small amount is needed and why the catalyst can be recovered unchanged at the end.

The answer that scoresAt a higher temperature the particles gain kinetic energy and move faster, so they collide more frequently. More importantly, each collision is more energetic, so a greater proportion of the collisions has energy greater than or equal to the activation energy. Both effects increase the number of effective collisions per second, so the rate increases.

The answer that scoresAt dynamic equilibrium the forward and reverse reactions are both still taking place, but at equal rates. Because the rates are equal, the concentrations of the reactants and products no longer change, so the amounts appear constant even though the reactions have not stopped.

The answer that scoresA reducing agent reduces another substance and is itself oxidised. It gives electrons to the other substance. In 2KI + Cl2 -> 2KCl + I2 the iodide ions are the reducing agent, because they reduce the chlorine to chloride ions, and the iodide ions themselves are oxidised to iodine.

The answer that scoresA weak acid is one that is only partially dissociated in aqueous solution, so most of the molecules stay whole and only a small fraction release H+ ions. Ethanoic acid is weak because of its incomplete dissociation, however concentrated the solution is. Hydrochloric acid is a strong acid because it dissociates completely, and it too can be either dilute or concentrated.

The answer that scoresAcid plus carbonate produces a salt, water and carbon dioxide: CaCO3 + 2HCl -> CaCl2 + H2O + CO2. Hydrogen is produced when an acid reacts with a metal, for example Mg + 2HCl -> MgCl2 + H2. Carbon dioxide turns limewater milky, while hydrogen gives a squeaky pop with a lighted splint.

The answer that scoresCopper(II) oxide is a basic oxide, because it is a metal oxide that reacts with acids to form a salt and water. An amphoteric oxide reacts with both acids and bases, and the two examples are aluminium oxide and zinc oxide. Zinc oxide reacts with sulfuric acid, ZnO + H2SO4 -> ZnSO4 + H2O, and also with sodium hydroxide, ZnO + 2NaOH -> Na2ZnO2 + H2O.

The answer that scoresA pH of 2 is strongly acidic. The scale runs from 0 to 14, and every step of one pH unit is a tenfold change in hydrogen ion concentration, so a solution of pH 2 has ten times the H+ concentration of one of pH 3. Only a pH close to 7 is weakly acidic or weakly alkaline.

The answer that scoresReactivity increases down Group I from lithium to sodium to potassium. The single outer electron is further from the nucleus and is shielded by more inner shells, so it is held less strongly and is lost more easily. Potassium therefore reacts fastest and most violently with water.

The answer that scoresIodine is less reactive than bromine, so it cannot displace bromine from potassium bromide. There would be no reaction and the colourless solution would stay colourless. Only a halogen higher up the group than bromine, such as chlorine, can displace it.

The answer that scoresSodium reacts quickly with oxygen and with water vapour in the air, forming a white layer of sodium oxide and sodium hydroxide on the surface. The oil keeps the air and its water vapour away from the metal, so the sodium stays shiny and unreacted.

The answer that scoresThe noble gases are unreactive because each atom has a full outer shell of electrons. Atoms react in order to gain a full outer shell, but a noble gas already has one, so it has no need to gain, lose or share electrons and forms no bonds. Helium has 2 outer electrons, neon 2,8 and argon 2,8,8.

The answer that scoresIron rusts when both oxygen and water are present. The oxygen comes from the air and the water can come from rain or from the humidity in the air. Together they react with the iron to form hydrated iron(III) oxide, Fe2O3.xH2O, which is the orange-brown flaky solid called rust. If either oxygen or water is excluded, the iron does not rust.

The answer that scoresIn a pure metal the atoms are all the same size and sit in regular layers, so the layers can slide over each other and the metal is soft. In an alloy the atoms of the different elements are different sizes, so the regular layers are distorted and can no longer slide over each other. That is why an alloy such as brass is harder and stronger than pure copper.

The answer that scoresZinc is more reactive than iron, so it corrodes in preference to the iron. If the zinc coating is scratched, the zinc loses electrons and is oxidised instead of the iron, so the iron is still protected. This is called sacrificial protection. The zinc coating also acts as a barrier, keeping out water and oxygen.

The answer that scoresAluminium is more reactive than carbon, so carbon cannot displace it from its oxide. Aluminium is therefore extracted by the electrolysis of purified bauxite dissolved in molten cryolite. Cryolite lowers the melting point to about 950 °C, and the reactions at the electrodes are Al3+ + 3e- -> Al at the cathode and 2O2- -> O2 + 4e- at the anode.

The answer that scoresAnhydrous copper(II) sulfate is white. When water is added it becomes hydrated and turns blue. The reverse change, blue to white, is what happens when the blue hydrated crystals are heated and the water is driven off again.

The answer that scoresClean, dry air is approximately 78% nitrogen and 21% oxygen, with the remainder a mixture of noble gases and carbon dioxide. Oxygen is the gas used in combustion and respiration, so its share is the smaller of the two.

The answer that scoresThe greenhouse gases you need to know are carbon dioxide and methane. Both absorb thermal energy radiated from the Earth's surface and re-emit it, so less energy is lost to space and the atmosphere warms. Acid rain is caused by sulfur dioxide from burning fossil fuels that contain sulfur compounds, and by oxides of nitrogen from car engines.

The answer that scoresSedimentation and filtration remove solids, carbon removes tastes and odours, and chlorination kills the harmful microbes. Each stage has one job, and the mark is given for naming the stage and stating what it removes.

The answer that scoresButane and methylpropane are structural isomers: they share the molecular formula C4H10 but have different structural formulae, CH3CH2CH2CH3 and CH3CH(CH3)CH3. The same is true of but-1-ene, CH3CH2CH=CH2, and but-2-ene, CH3CH=CHCH3, which are both C4H8.

The answer that scoresPetroleum is a mixture of hydrocarbons with different boiling points. In the fractionating column each fraction condenses over a range of heights because it contains many different hydrocarbons with similar, but not identical, boiling points. So every fraction is still a mixture, not a pure compound.

The answer that scoresFermentation is carried out at 25-35 °C in the absence of oxygen. The yeast provides enzymes that break glucose down into ethanol and carbon dioxide, C6H12O6 -> 2C2H5OH + 2CO2. Air must be kept out so that the ethanol is not oxidised to ethanoic acid, and the temperature must stay below about 40 °C so that the enzymes are not denatured.

The answer that scoresMost plastics are unreactive and not biodegradable, so they persist for a very long time. In landfill sites they take up land and remain largely unchanged; in the oceans they accumulate and harm wildlife that swallows or becomes tangled in them; and burning them is not a safe solution, because incomplete combustion can release toxic gases such as carbon monoxide. Recycling, and using condensation polymers such as PET that can be converted back into monomers, reduces the problem.

The answer that scoresThe acid is placed in a burette and added drop by drop to a known volume of alkali measured with a volumetric pipette. The burette is read to the nearest 0.1 cm3, and the volume used is the final reading minus the initial reading.

The answer that scoresRf = distance travelled by the substance / distance travelled by the solvent = 3.6 / 9.0 = 0.40. There are no units, because both distances are in cm and the units cancel out.

The answer that scoresA saturated solution contains the maximum concentration of a solute that can dissolve in the solvent at a specified temperature. Adding more solute at that temperature leaves it undissolved at the bottom. Warming the solution usually lets more solute dissolve, because solubility increases with temperature.

The answer that scoresAmmonia turns damp red litmus paper blue. A glowing splint is used for oxygen, which relights it, and a lighted splint is used for hydrogen, which gives a squeaky pop.

The answer that scoresThe cylinder should be standing on a level bench and the eye should be level with the mark, looking straight at the scale. The volume is read at the bottom of the meniscus, the lowest point of the curved water surface. Read this way, the value is the true volume of liquid in the cylinder.

The answer that scoresTime 20 complete oscillations and divide the total time by 20 to get the period. The total time is much longer, so the reaction-time error is only a small fraction of it, and the period comes out far more accurately. Repeat the measurement and average the results to reduce random error further.

The answer that scoresresultant = sqrt(6.0 x 6.0 + 8.0 x 8.0) = sqrt(36 + 64) = sqrt(100) = 10 N. The direction is found from tan theta = 6.0 / 8.0 = 0.75, so the resultant is 10 N at 37 deg to the 8.0 N direction, between the two original forces.

The answer that scoresSpeed is a scalar quantity, because it has magnitude only: 8 m/s tells you how fast the car moves but not which way. Velocity is a vector quantity, because it is speed in a stated direction, for example 8 m/s due north. A car going round a roundabout at a steady speed has a changing velocity, because its direction is changing.

The answer that scoresHeating transfers energy to the gas particles, so their average kinetic energy increases and they move faster. The particles themselves stay the same size; it is the average distance between them that increases, so the gas expands. If the gas is trapped in a container of fixed volume, the faster particles collide with the walls more often and with more force, and the pressure rises instead.

The answer that scoresHeating gives the particles more kinetic energy, so they move faster. They therefore collide with the walls of the container more frequently, and each collision exerts a larger force on the wall. The total force per unit area on the walls increases, so the pressure of the gas rises.

The answer that scoresThe smoke particles are bombarded by the much smaller, much faster-moving molecules of the air. The collisions are random and uneven, so at any instant the pushes on one side do not balance the pushes on the other, and the net force changes direction from moment to moment. The smoke particle therefore follows a random zigzag path. This is Brownian motion, and it is evidence that the air molecules are in constant random motion.

The answer that scoresT (in K) = theta (in °C) + 273, so rearranging gives theta (in °C) = T (in K) - 273. For 25 K: theta = 25 - 273 = -248 °C. A negative answer is possible in degrees Celsius but never in kelvin, because 0 K is absolute zero, the lowest temperature there is.

The answer that scoresWhen a wave crosses into a new medium its speed changes, so the wave bends. Because v = f x wavelength and the frequency is fixed by the source, the wavelength must change by the same factor as the speed. If the wave slows down the wavelength becomes shorter, but the frequency stays the same.

The answer that scoresThe amplitude is the maximum displacement of a point from its rest position, measured from the middle line of the wave up to a crest or down to a trough. The distance from a crest to the next trough is twice the amplitude, not the amplitude itself.

The answer that scoresA wave transfers energy from one place to another without transferring matter. In a water wave the water particles move up and down and end where they began, so it is the energy of the wave that travels forward. A floating ball bobs up and down as a wave passes but is not carried along with it.

The answer that scoresDiffraction is the spreading of waves as they pass through a gap. The spreading is greatest when the width of the gap is similar to or smaller than the wavelength. If the gap is much wider than the wavelength the waves pass through with very little spreading, so widening the gap reduces diffraction.

The answer that scoresCopper is a metal, but it is non-magnetic, so a magnet does not attract it at all. The magnetic materials are iron, steel, cobalt and nickel. A magnet attracts these because their atoms behave like tiny magnets that can be lined up by the field, while the atoms of copper, aluminium and brass cannot be lined up in this way.

The answer that scoresThe compass needle is a small magnet that is free to turn. The magnetic pole of the Earth near the geographic north is a magnetic south pole, so it attracts the north-seeking pole of the needle. That is why the needle comes to rest pointing north-south, and why the end that points north is called the north pole of the magnet.

The answer that scoresOutside the magnet the field lines are drawn from the N pole to the S pole, because the direction of the field at a point is the direction of the force on a north pole placed there. The lines are closest together at the two poles and further apart in the space around the middle of the magnet, and the spacing of the lines shows the relative strength of the field.

The answer that scoresThe crane uses an electromagnet, which is a coil of wire wound on a soft iron core. It can be switched on and off with the current, so the crane can pick up the scrap and then drop it, and its strength can be changed to suit the load. An electromagnet can also be made much stronger than any permanent magnet, because its strength increases with the current and with the number of turns of wire.

The answer that scoresAn alpha particle is a helium nucleus with a relative mass of 4 and a relative charge of +2. An electron is about 1840 times lighter than a proton, so it cannot deflect an alpha particle through a large angle. The particles that came almost straight back had passed very close to a very small, dense, positively charged nucleus, which repelled them strongly because both the nucleus and the alpha particle are positive. Those rare large deflections are the evidence that the nucleus is small, massive and positive, while the many undeflected particles show that the atom is mostly empty space.

The answer that scoresIsotopes are atoms of the same element with the same number of protons but different numbers of neutrons. Chlorine-35 and chlorine-37 both have 17 protons, so both are chlorine. Chlorine-35 has 35 - 17 = 18 neutrons and chlorine-37 has 37 - 17 = 20 neutrons. Because both have 17 electrons in a neutral atom, they have the same chemical properties, but their masses are different.

The answer that scoresA negative ion is formed when an atom gains one or more electrons. A chlorine atom has proton number 17, so it has 17 protons and 17 electrons; when it gains one electron it has 17 protons and 18 electrons, so there is one more negative charge than positive charge and the ion has a charge of 1-, written Cl-. The number of protons is unchanged when an ion forms.

The answer that scoresNuclear fission is the splitting of a heavy nucleus, such as uranium-235, into two smaller nuclei after it absorbs a neutron. It releases energy and more neutrons, so a chain reaction can build up, and this is the process used in a nuclear power station. Nuclear fusion is the joining of two light nuclei, such as hydrogen nuclei, to form a heavier nucleus such as helium, and it releases even more energy per kilogram of fuel. Fusion only happens at extremely high temperatures and pressures, as in the core of the Sun, so no fusion power station yet produces more energy than it uses.

The answer that scoresThe Earth's axis is tilted, and the tilt keeps pointing in the same direction as the Earth orbits the Sun. When the northern hemisphere leans towards the Sun, the Sun's rays strike it more directly and the days are longer, so it is summer there. When it leans away, the rays arrive at a shallower angle and the days are shorter, so it is winter. Six months later the opposite hemisphere is tilted towards the Sun, so the seasons repeat once every orbit, about 365 days.

The answer that scoresThe Moon is lit by the Sun, so half of it is always lit and half is always dark. As the Moon orbits the Earth once a month, the fraction of the lit half that faces the Earth changes. At new moon the lit half faces away from us, at full moon it faces us completely, and in between we see crescents, half moons and gibbous shapes. The cycle repeats every orbit, which takes approximately one month.

The answer that scoresConvert the period first: T = 365 x 24 x 60 x 60 = 3.15 x 107 s. Then substitute: v = 2 x pi x r / T = (2 x 3.14 x 1.5 x 1011) / (3.15 x 107) = (9.42 x 1011) / (3.15 x 107) = 3.0 x 104 m/s. Always convert T to seconds before substituting, and check that the answer is a sensible size.

The answer that scoresThe Sun contains most of the mass of the Solar System, so its gravitational field is by far the strongest. The force that keeps a planet in its orbit is the gravitational attraction of the Sun, which pulls the planet towards the Sun and provides the force needed for it to follow a curved path. The same idea keeps moons in orbit around their planets and satellites in orbit around the Earth.