2024 CCEA GCSE Science Double Award 1370 模擬試題連答案詳解

(a) A valid four-organism food chain drawn from the information given, with arrows pointing in the direction of energy flow (from what is eaten to what eats it): Oak tree → caterpillar → blue tit → tawny owl.

(b) A primary consumer is an organism that eats producers directly; a secondary consumer is an organism that eats a primary consumer. From the information given, caterpillars and wood mice are primary consumers (they eat the oak tree/acorns directly). Blue tits, which eat caterpillars, are secondary consumers. Tawny owls, when eating wood mice or blue tits, are also acting as secondary consumers (since wood mice and blue tits are themselves primary or secondary consumers respectively) - more precisely, tawny owls eating wood mice (a primary consumer) makes the owl a secondary consumer, while owls eating blue tits (a secondary consumer) makes the owl a tertiary consumer in that particular chain; for the purposes of this answer, blue tits and tawny owls (when eating wood mice) are the organisms that function as secondary consumers within this food web.

(c) The arrows in a food chain or food web point from the organism being eaten to the organism that eats it, and represent the transfer of substances (such as carbon and nitrogen) and energy through the ecosystem as feeding takes place.

(d) A producer is an organism that is able to make its own organic/food molecules, rather than obtaining them by feeding on other organisms; producers form the base of every food chain. Oak trees, as producers, obtain the energy that enters this food web by photosynthesis: chlorophyll in the leaves absorbs light energy from the Sun, which is used to convert carbon dioxide and water into glucose (and, ultimately, starch and other food molecules), storing the Sun's light energy as chemical energy within the tree's tissues.

(e) Reason 1: a large proportion of the energy taken in by the wood mouse is released as heat during respiration and lost to the surroundings, rather than being stored within its body tissues. Reason 2: some parts of the wood mouse's body (such as bones, fur or other indigestible material) are not eaten by the tawny owl, or are eaten but not digested and are egested as waste (in owl pellets), so this energy is not transferred into the owl's own tissues. Reason 3: the wood mouse uses much of the energy it obtains from feeding for its own life processes - such as movement, growth, and maintaining body functions - before it is ever eaten, meaning this energy has already been used/dissipated and is not available to be passed on to the tawny owl.

(a) [2] correct four-organism food chain in the correct order with arrows in the correct direction (allow [1] if organisms correct but arrows/order wrong). (b) [1] blue tit correctly identified as a secondary consumer + [1] tawny owl correctly identified as acting as a secondary consumer (when eating wood mice). (c) [1] represents transfer of energy + [1] represents transfer of substances (carbon/nitrogen) / direction of feeding. (d) [1] producer correctly defined (makes own food/organic molecules) + [1]-[2] correct explanation that oak trees photosynthesise, using chlorophyll to absorb light energy and convert it to stored chemical energy. (e) [1] mark for each of three valid, distinct reasons, up to [3]: energy lost as heat via respiration; energy lost in undigested/uneaten material (egestion); energy used by the wood mouse for its own life processes (movement, growth) before being eaten. Maximum [12] overall.

A high-scoring answer explains the composition and function of all three biological molecules with accurate, specific detail.

Carbohydrates are made up of simple carbohydrates (sugars such as glucose and lactose) and complex carbohydrates (including cellulose, starch and glycogen). Simple sugars such as glucose are used directly by cells as a source of energy, released through respiration; more complex carbohydrates such as starch and glycogen act as storage forms of carbohydrate, in plants and animals respectively, which can be broken down to release glucose when needed. Cellulose forms an important structural component of plant cell walls.

Fats/lipids, including oils, are made up of fatty acids and glycerol joined together. Fats are a concentrated source of energy, providing more energy per gram than carbohydrate, and are used by the body as a longer-term energy store, typically stored in adipose tissue; fats also provide insulation and protect delicate organs.

Proteins are made up of chains of amino acids joined together. Unlike carbohydrates and fats, proteins are used mainly as structural and functional molecules within cells, rather than primarily as an energy source; for example, proteins form structural components of cells and tissues (such as muscle), and functional proteins include enzymes, which catalyse the chemical reactions needed for life.

A thorough answer draws a clear distinction between the two main energy-storage/energy-source molecules (carbohydrate and fat) and the primarily structural/functional role of protein, using accurate specialist vocabulary throughout (such as glucose, glycogen, fatty acids, glycerol, amino acids and enzymes).

Banded mark scheme (6 marks total).

Band C [1]-[2]: Basic, limited response; may address only one biological molecule, or state composition/function without linking the two; weak use of specialist vocabulary; some errors in spelling/grammar that hinder meaning.

Band B [3]-[4]: Addresses at least two of the three biological molecules with reasonable accuracy, covering both composition and function; developing use of specialist vocabulary (e.g. amino acids, fatty acids, glycogen); reasonably organised.

Band A [5]-[6]: Addresses all three biological molecules (carbohydrate, fat/lipid, protein) accurately, clearly explaining composition (e.g. sugars/starch/glycogen; fatty acids and glycerol; amino acids) and function (energy source/storage vs structural/functional) for each; wide range of accurate specialist vocabulary; clear coherence, spelling and grammar throughout.

(a) Iodine solution changes from yellow-brown to blue-black in the presence of starch.

(b) The temperature at which an enzyme-catalysed reaction proceeds at its maximum rate is called the optimum temperature.

(c) As the temperature increases from 10°C to 40°C, both the enzyme (amylase) and substrate (starch) particles gain more kinetic energy and move around more quickly. This increased movement means that enzyme and substrate particles collide more frequently, and a greater proportion of these collisions have sufficient energy to result in a successful reaction (the substrate binding to the active site and being broken down); both effects increase the rate of reaction as temperature rises towards the optimum.

(d) Above the optimum temperature, the additional heat energy causes the bonds holding the enzyme's tertiary (3D) structure together to break; this causes an irreversible change to the shape of the enzyme's active site, a process called denaturation. Once the active site's shape has changed, the substrate (starch) molecule can no longer fit into it, so the enzyme can no longer catalyse the breakdown of starch, causing the sharp fall in rate of reaction seen between 40°C and 60°C, with almost no reaction at 60°C because the enzyme is now fully denatured.

(e) The lock and key model describes how an enzyme's active site has a specific three-dimensional shape that is complementary to the shape of one particular substrate, in the same way that a specific key is needed to fit a specific lock. Amylase's active site has a shape that precisely fits starch molecules, allowing starch to bind to the active site so that it can be broken down (into smaller sugar molecules). Protein molecules have a completely different molecular shape to starch, and so cannot fit into amylase's active site (just as the wrong key cannot open a lock); this explains why amylase is specific to starch and cannot break down protein (protein is instead broken down by a different enzyme, protease, whose active site is shaped to fit protein).

(a) [1] blue-black. (b) [1] optimum (temperature). (c) [1] particles gain kinetic energy/move faster + [1] collide more frequently + [1] more successful/higher-energy collisions, increasing rate. (d) [1] enzyme becomes denatured above optimum + [1] high temperature causes irreversible change to shape of active site + [1] substrate can no longer fit/bind, so enzyme can no longer catalyse the reaction. (e) [1] active site has a specific shape complementary to one substrate (like a lock and key) + [1] amylase's active site shape fits starch, allowing it to bind and be broken down + [1] protein has a different shape that does not fit amylase's active site, so it is not broken down. Maximum [11] overall.

(a) The plant hormone responsible for phototropism (the growth response of a plant shoot towards light) is auxin.

(b) Auxin is produced at the tip of the shoot.

(c) Step 1: auxin, produced at the shoot tip, moves down the shoot. Step 2: when the shoot is exposed to light from only one direction (unidirectional light), the auxin becomes unevenly distributed within the shoot, moving towards and accumulating in a higher concentration on the shaded side of the shoot than on the side facing the light. Step 3: auxin causes the elongation (lengthening) of cells; because there is a higher concentration of auxin on the shaded side, the cells on that side elongate more than the cells on the illuminated side. Step 4: this differential (uneven) growth, with cells on the shaded side becoming longer than cells on the illuminated side, causes the shoot to bend towards the light, maximising the light available for photosynthesis.

(a) [1] auxin. (b) [1] produced at the tip of the shoot. (c) 1 mark per valid step, up to [3]: auxin moves down the shoot from the tip [1]; unidirectional light causes auxin to be unevenly distributed / more auxin accumulates on the shaded side [1]; higher auxin concentration causes greater cell elongation on the shaded side, causing the shoot to bend towards the light [1]. Maximum [5] overall.

(a) The word equation for anaerobic respiration in yeast is: glucose → energy + alcohol (ethanol) + carbon dioxide.

(b) As temperature increases from 15°C to 35°C, the number of bubbles produced per minute increases (from 8 to 35), showing that the rate of anaerobic respiration in the yeast increases as temperature increases over this range.

(c) Between 35°C and 45°C, the number of bubbles produced falls sharply (from 35 to 6). This is because the enzymes within the yeast cells that catalyse respiration have an optimum temperature; once the temperature rises above this optimum (which the data suggests is close to 35°C), the enzymes begin to denature - the high temperature causes an irreversible change to the shape of their active sites, meaning they can no longer catalyse the reactions of respiration as effectively, causing the sharp fall in the rate of carbon dioxide (bubble) production.

(d) To ensure a fair test, the student should have kept other variables that could affect the rate of respiration constant across all four temperatures, for example using the same concentration and volume of glucose solution, and the same mass or volume of yeast, in each test tube.

(a) [1] glucose identified as the reactant + [1] energy, alcohol/ethanol AND carbon dioxide correctly identified as the products (accept if worded as 'glucose → energy + ethanol + carbon dioxide'). (b) [1] correctly describes an increase in bubbles/rate as temperature increases over this range. (c) [1] enzymes become denatured above the optimum temperature + [1] correctly links this to reduced/no catalytic activity, reducing the rate of respiration/gas production. (d) [1] any valid controlled variable, e.g. concentration/volume of glucose solution, mass/volume of yeast. Maximum [6] overall.

(a) The process by which nitrogen gas is converted into nitrogen-containing compounds usable by plants is called nitrogen fixation.

(b) Nitrogen-fixing bacteria that live in a mutualistic relationship within the roots of leguminous plants are housed in swellings on the roots called root nodules.

(c) Plants need a supply of nitrogen-containing compounds (absorbed from the soil as nitrates through their roots) because nitrogen is an essential component of amino acids, which are joined together to make proteins; plants need protein for growth and to build structural and functional molecules within their cells, including enzymes.

(d) When decomposing microorganisms (saprophytic bacteria and fungi) break down the proteins in dead organisms, faeces and other organic waste, they release ammonia as a by-product; this process is part of decomposition (sometimes specifically termed ammonification).

(e) In a waterlogged, anaerobic (oxygen-poor) soil, conditions favour denitrifying bacteria, which convert nitrates in the soil back into nitrogen gas, which is then lost from the soil into the atmosphere; this reduces the amount of nitrogen available for plants to absorb. In a well-aerated, aerobic (oxygen-rich) soil, conditions instead favour nitrifying bacteria, which require oxygen to convert ammonia (from decomposition) first into nitrites and then into nitrates; because nitrates are the form of nitrogen that plants can readily absorb through their roots, well-aerated soil therefore makes more nitrogen available for plant growth than waterlogged soil.

(a) [1] nitrogen fixation. (b) [1] root nodules. (c) [1] nitrogen used to make proteins/amino acids + [1] needed for growth/structural or functional molecules (e.g. enzymes). (d) [1] decomposition (accept ammonification/putrefaction). (e) [1] waterlogged/anaerobic soil favours denitrifying bacteria, converting nitrates back to nitrogen gas (reducing nitrogen available) + [1] aerobic/well-aerated soil favours nitrifying bacteria, converting ammonia to nitrates (making more nitrogen available for uptake). Maximum [7] overall.

(a) A root hair cell has a long, extended (hair-like) projection that grows out into the soil. This extended shape significantly increases the surface area of the cell that is in contact with the surrounding soil water and soil particles, increasing the rate at which water and mineral ions (such as nitrate) can be absorbed into the cell.

(b) Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient, and does not require energy. In this scenario, nitrate ions are being absorbed into the root hair cell even though the concentration of nitrate is already higher inside the cell than in the soil outside; because this movement is against (up) the concentration gradient, it cannot be explained by diffusion, which can only move substances down a concentration gradient.

(c) The process responsible is active transport (sometimes called active uptake). Active transport requires energy to move substances against a concentration gradient; this energy is supplied by respiration taking place within the root hair cell (respiration releases energy from glucose, which is used to power the active transport of mineral ions such as nitrate into the cell).

(a) [1] root hair cell has an extended/elongated (hair-like) shape + [1] correctly links this to increased surface area for absorption. (b) [1] correctly explains diffusion only moves substances down (from high to low) a concentration gradient, but nitrate is moving against/up the gradient here. (c) [1] correctly names active transport/active uptake + [1] correctly identifies that the energy for this process comes from (cell) respiration. Maximum [5] overall.

(a) Decrease in oxygen concentration = 9.5 - 3.8 = 5.7 mg/dm³. Percentage decrease = (decrease ÷ original value) × 100 = (5.7 ÷ 9.5) × 100 = 60%. Final answer: the dissolved oxygen concentration fell by 60%.

(b) Step 1: the nitrates from the fertiliser run-off act as a nutrient, stimulating rapid growth of aquatic plants and algae near the water's surface, sometimes called an algal bloom. Step 2: this dense growth of algae and plants near the surface blocks light from reaching plants deeper in the pond; deeper plants (and, eventually, the algae themselves once nutrients are depleted) die due to lack of light and subsequent nitrate depletion. Step 3: aerobic microorganisms (bacteria and fungi) feed on and decompose the large amount of dead plant and algal material that has accumulated. Step 4: because this decomposition is an aerobic process, the microorganisms use up dissolved oxygen from the water as they respire, causing the dissolved oxygen concentration of the pond to fall.

(c) With dissolved oxygen concentration greatly reduced, aquatic animals such as fish and invertebrates that rely on dissolved oxygen for aerobic respiration may not have enough oxygen available; this can cause them to suffocate and die, potentially leading to a significant loss of animal life within the pond.

(a) [1] correct method (decrease ÷ original × 100) + [1] correct substitution (5.7 ÷ 9.5 × 100) + [1] correct final answer (60%). (b) 1 mark per correct stage in the correct sequence, up to [4]: nitrates stimulate growth of aquatic plants/algae (algal bloom) [1]; this blocks light to other/deeper plants, causing them to die (from lack of light/nitrate depletion) [1]; aerobic microorganisms/bacteria decompose the dead plant and algal material [1]; this decomposition uses up dissolved oxygen from the water [1]. (c) [1]-[2] valid consequence for aquatic animals, e.g. reduced oxygen available for aerobic respiration [1] + animals (fish/invertebrates) suffocate/die [1]. Maximum [9] overall.

(a) At very low light intensity, there is insufficient light energy available to drive photosynthesis at a significant rate, so the rate of photosynthesis is very low. However, the pondweed's cells continue to respire (releasing carbon dioxide as a waste product) at their usual rate, regardless of light intensity. Because the rate of respiration (producing CO2) is greater than the rate of photosynthesis (absorbing CO2) under these conditions, there is a net release of carbon dioxide into the surrounding water, raising the CO2 concentration and turning the indicator yellow.

(b) At the higher light intensity described, the rate of photosynthesis has increased to the point where it exactly matches the rate of respiration; this light intensity is known as the compensation point. At the compensation point, the carbon dioxide being released by respiration is being absorbed and used by photosynthesis at exactly the same rate, so there is no overall (net) change in the carbon dioxide concentration of the water, meaning the indicator remains at its normal (red) colour.

(c) At high light intensity, there is ample light energy available, so the rate of photosynthesis increases well beyond the rate of respiration. The pondweed is now absorbing carbon dioxide (for photosynthesis) at a much faster rate than respiration is releasing it, resulting in a net decrease in the carbon dioxide concentration of the surrounding water, which turns the indicator purple.

(d) Other factors that could be investigated for their effect on the rate of photosynthesis include carbon dioxide concentration (for example by adding different amounts of sodium hydrogencarbonate to the water) and temperature (by placing the test tubes in water baths at different temperatures), both of which are known limiting factors of photosynthesis alongside light intensity.

(a) [1] photosynthesis rate very low/negligible at very low light + [1] respiration continues at its normal rate + [1] respiration (releasing CO2) exceeds photosynthesis (absorbing CO2), so CO2 concentration rises, turning indicator yellow. (b) [1] correct use of the term compensation point, where rate of photosynthesis = rate of respiration + [1] correct explanation that CO2 released and absorbed are balanced, so no net change/indicator stays red. (c) [1] photosynthesis rate greatly exceeds respiration rate at high light intensity + [1] correct explanation that CO2 is absorbed faster than it is produced, so CO2 concentration falls, turning indicator purple. (d) [1] mark for each valid factor, up to [2]: carbon dioxide concentration; temperature. Maximum [9] overall.

(a)(i) Magnesium forms a 2+ ion (Mg²⁺) and oxide forms a 2- ion (O²⁻); since the charges are equal and opposite, they combine in a 1:1 ratio, giving the formula MgO. (ii) Carbon dioxide consists of one carbon atom combined with two oxygen atoms, giving the formula CO2. (iii) Calcium forms a 2+ ion (Ca²⁺) and chloride forms a 1- ion (Cl⁻); two chloride ions are needed to balance the charge of one calcium ion, giving the formula CaCl2.

(b) The unbalanced equation Mg + O2 → MgO has one oxygen atom on the left of the arrow existing as part of an O2 molecule (2 oxygen atoms) but only one oxygen atom in MgO on the right; to balance the oxygen atoms, 2 MgO must be produced, which in turn requires 2 Mg to react: 2Mg + O2 → 2MgO. Checking: left-hand side has 2 Mg atoms and 2 O atoms; right-hand side has 2 Mg atoms and 2 O atoms - the equation is now balanced.

(c) When magnesium burns in oxygen, it burns with a characteristic bright white flame/light, and a white solid (magnesium oxide powder) is produced.

(a) [1] mark for each correct formula: (i) MgO; (ii) CO2; (iii) CaCl2. (b) [1] mark for correctly placing a '2' in front of MgO; [1] mark for correctly placing a '2' in front of Mg (both required for a fully balanced equation, allow [1] for a partially correct attempt showing understanding of balancing). (c) [1] mark for a valid observation, e.g. bright white flame/light, white solid/powder produced. Maximum [6].

(a) Mendeleev arranged the known elements of his time mainly in order of increasing relative atomic mass, placing them into rows and columns so that elements with similar chemical properties fell into the same vertical column (group).

(b) Because Mendeleev arranged elements by both atomic mass and their chemical properties, he noticed that the known pattern of properties was sometimes interrupted; rather than force an element with clearly different properties into the wrong position, he left a gap in the table at that point, assuming an element that had not yet been discovered should occupy it. By looking at the properties of the elements immediately above, below and beside a gap in the same group and period, Mendeleev was able to predict with good accuracy what the properties (such as density, reactivity and the formulae of compounds it would form) of the missing, as yet undiscovered element would be; when several of these elements (such as gallium and germanium) were later discovered, their actual properties closely matched Mendeleev's predictions, which was strong evidence in favour of his table.

(c) Mendeleev's table was arranged in order of increasing relative atomic mass, whereas the modern Periodic Table is arranged in order of increasing atomic (proton) number; in addition, the modern table includes entire groups of elements, such as the noble gases, that were completely unknown at the time Mendeleev constructed his table.

(d) When a small piece of potassium is added to water, it reacts vigorously: it moves rapidly across the surface of the water, fizzing as hydrogen gas is rapidly produced; the heat released by the reaction is often enough to melt the potassium into a small, shiny ball, and may ignite the hydrogen gas produced, causing the potassium to burn with a characteristic lilac (pale purple) flame.

(e) Unbalanced: K + H2O → KOH + H2. Balancing hydrogen: there are 2 H atoms in H2O (as written) but water needs to supply enough H atoms for one H2 molecule (2 H atoms) as well as one H atom in KOH; using 2 H2O gives 4 H atoms on the left, enough to form 2 KOH (2 H atoms) and 1 H2 (2 H atoms) on the right; this then requires 2 K on the left to balance 2 KOH on the right. Balanced equation: 2K + 2H2O → 2KOH + H2. Check: left-hand side has 2 K, 4 H, 2 O; right-hand side has 2 K, 2+2=4 H, 2 O - balanced.

(f) All Group 1 elements have just one electron in their outer shell. Going down the group, each successive element has an additional electron shell, meaning the single outer shell electron is positioned further away from the positively charged nucleus. Because this outer electron is further from the nucleus (and increasingly shielded by inner electron shells), the force of attraction between the nucleus and the outer electron is weaker, meaning the outer electron is more easily lost when the atom reacts; this explains why reactivity increases going down Group 1.

(a) [1] arranged mainly by increasing relative atomic mass (with elements of similar properties grouped together). (b) [1] gaps left where properties suggested an undiscovered element + [1] properties of surrounding known elements used to predict the properties of the missing element. (c) [1] any valid difference, e.g. modern table ordered by atomic/proton number (not atomic mass); modern table includes noble gases. (d) [1] mark for each valid observation, up to [2]: moves/skates across the surface; fizzes/produces gas (hydrogen); may ignite/burn with a lilac flame; melts into a ball. (e) [1] correct balancing of hydrogen/potassium hydroxide (2KOH, H2) + [1] correct balancing of potassium and water (2K, 2H2O) for a fully balanced equation. (f) [1] outer shell electron is further from the nucleus/more shielded going down the group + [1] correctly links this to weaker attraction/electron more easily lost, increasing reactivity. Maximum [10] overall.

(a) A pure substance consists of only one element or one compound, with no other substances mixed in; a defining property of a pure substance is that it has one specific, fixed melting point and one specific, fixed boiling point.

(b) If a liquid sample is impure (a mixture), it will not boil (or melt) sharply at one fixed temperature; instead, it will boil over a range of temperatures, and/or its boiling point will differ from the known, published boiling point of the pure substance it is claimed to be. Measuring the melting or boiling point and comparing it with the known value (and range) for the pure substance is a way to test purity.

(c) Step 1: the ethanol-water mixture is heated in a flask connected to a fractionating column. Step 2: as the mixture is heated, the temperature rises until it reaches the boiling point of ethanol (78°C), which is lower than the boiling point of water (100°C), so the ethanol evaporates and turns to vapour before the water does. Step 3: the ethanol vapour rises up through the fractionating column and passes into a condenser. Step 4: in the condenser, the ethanol vapour is cooled and condenses back into liquid ethanol, which drips out and is collected separately, while the water, which has not yet reached its own (higher) boiling point, remains behind as a liquid in the original flask.

(a) [1] correct definition (single element/compound with a fixed melting and boiling point). (b) [1] valid evidence, e.g. boils/melts over a range of temperatures rather than at a single fixed point. (c) 1 mark per valid step, up to [3]: mixture is heated + ethanol boils/evaporates first because it has the lower boiling point [1]; ethanol vapour rises up the fractionating column into a condenser [1]; ethanol vapour condenses/cools back to a liquid and is collected separately, while water (higher boiling point) remains behind [1]. Maximum [5] overall.

A high-scoring answer to this multi-part question demonstrates accurate knowledge across observation, equations, energy changes, ionic theory and the pH scale, using clear, well-organised scientific language.

(a) When carbon dioxide gas is bubbled through limewater (calcium hydroxide solution), the limewater turns from colourless to milky/cloudy; this is the standard test used to identify carbon dioxide gas.

(b) The word equation is calcium carbonate + hydrochloric acid → calcium chloride + water + carbon dioxide. To balance the symbol equation, since there are 2 chlorine atoms in CaCl2, 2 HCl are needed to supply them on the left-hand side: CaCO3 + 2HCl → CaCl2 + H2O + CO2. Checking: left-hand side has 1 Ca, 1 C, 3 O (in CaCO3) + 2 H, 2 Cl (in 2HCl) = 1 Ca, 1 C, 3 O, 2 H, 2 Cl; right-hand side has 1 Ca, 2 Cl (CaCl2) + 2 H, 1 O (H2O) + 1 C, 2 O (CO2) = 1 Ca, 2 Cl, 2 H, 3 O, 1 C - the equation is balanced.

(c) This is an exothermic reaction, meaning heat energy is given out to the surroundings as the reaction proceeds. This could be confirmed experimentally by placing a thermometer in the reaction mixture and recording the temperature before adding the reactants together and then at intervals during the reaction; an increase in temperature would confirm that the reaction is exothermic.

(d) During a neutralisation reaction, the hydrogen ions (H+) present in the acid react with the hydroxide ions (OH-) present in the alkali; these oppositely-charged ions combine chemically to form water molecules, which are neutral (neither acidic nor alkaline). This can be represented by the ionic equation: H+(aq) + OH-(aq) → H2O(l).

(e) The pH scale gives a measure of how acidic or alkaline a solution is, and directly reflects the concentration of H+ ions present in solution - the lower the pH (closer to 0), the higher the concentration of H+ ions, and the more strongly acidic the solution. A strong acid, such as hydrochloric acid, is fully ionised when dissolved in water, meaning essentially every acid molecule releases its H+ ion(s) into solution; this produces a high concentration of H+ ions and therefore gives a low pH value (typically around pH 0-1 for a concentrated strong acid). A weak acid, such as ethanoic acid, is only partially ionised in water - only a small proportion of the acid molecules release their H+ ions, with most remaining as un-ionised molecules; this produces a lower concentration of H+ ions than an equally concentrated strong acid, and therefore gives a higher pH value (though it will still be below 7, since the solution remains acidic overall). Comparing the measured pH values of a strong acid and a weak acid of the same concentration therefore allows their relative strength to be assessed - the acid with the lower pH is the stronger acid.

Banded mark scheme (12 marks total across all parts; Band mark scheme applies specifically to part (e), which is assessed for QWC, with the remaining parts marked on a point basis).

(a) [1] limewater turns milky/cloudy + [1] carbon dioxide correctly named. (b) [1] correct balancing of chlorine/calcium chloride (2HCl, CaCl2) + [1] fully balanced equation confirmed (H2O, CO2 correctly balance). (c) [1] exothermic correctly stated + [1] valid method to confirm (e.g. thermometer records temperature rise). (d) [1] H+ ions from acid + [1] react with OH- ions from alkali to form water + [1] correct ionic equation H+(aq) + OH-(aq) → H2O(l).

(e) QWC Band C [1]: basic reference to pH and acid strength, e.g. states strong acids have lower pH, with little/no explanation in terms of ionisation; weak use of specialist vocabulary. Band B [2]: reasonable explanation that strong acids are fully ionised and weak acids are partially ionised, linked to pH, but underdeveloped comparison and/or no valid named weak acid; developing specialist vocabulary. Band A [3]: clear, accurate explanation that a strong acid is fully ionised (producing more H+ ions, lower pH) while a weak acid is only partially ionised (producing fewer H+ ions, higher pH) at the same concentration, with a valid named weak acid (e.g. ethanoic acid, carbonic acid) given; accurate specialist vocabulary and clear expression.

Maximum [12] overall.

(a) A proton has a relative charge of +1 and a relative mass of 1. A neutron has a relative charge of 0 and a relative mass of 1. An electron has a relative charge of -1 and a relative mass that is very small compared with a proton or neutron (approximately 1/1840 of the mass of a proton, often treated as negligible).

(b) Isotopes are atoms of the same element - meaning they have the same number of protons (the same atomic number) - but which have a different number of neutrons, giving them a different mass number.

(c) The number of neutrons in an atom = mass number - atomic (proton) number. For chlorine-35: neutrons = 35 - 17 = 18. For chlorine-37: neutrons = 37 - 17 = 20.

(d) Relative atomic mass is calculated as the weighted mean of the mass numbers of the isotopes present, based on their abundance. Method: Ar = (mass number of isotope 1 × its % abundance) + (mass number of isotope 2 × its % abundance), all divided by 100. Substitution: Ar = (35 × 75) + (37 × 25), divided by 100. Calculation: Ar = (2625 + 925) ÷ 100 = 3550 ÷ 100 = 35.5. Final answer: Ar(Cl) = 35.5 (to 1 decimal place), which matches the accepted relative atomic mass of chlorine shown on the Periodic Table.

(a) [1] mark for each particle's charge and mass correctly stated, up to [3] (proton +1/1; neutron 0/1; electron -1/very small). (b) [1] correct definition of isotope (same atomic/proton number, different number of neutrons/mass number). (c) [1] chlorine-35 has 18 neutrons + [1] chlorine-37 has 20 neutrons. (d) [1] correct method (weighted mean using abundances) + [1] correct substitution ((35×75 + 37×25) ÷ 100) + [1] correct final answer (35.5). Error carried forward applies if part (c) is incorrect but the method in part (d) is otherwise correctly applied. Maximum [7] overall.

(a) Sodium chloride has a giant ionic lattice structure, in which oppositely charged sodium ions (Na+) and chloride ions (Cl-) are held together throughout the whole structure by strong electrostatic forces of attraction, known as ionic bonds, acting in all directions. Because these ionic bonds are strong and extend throughout the entire giant lattice, a very large amount of energy is required to overcome them and separate the ions, which is why sodium chloride has a high melting point. In the solid state, the ions are held tightly in fixed positions within the lattice by these strong ionic bonds and are not free to move, so there are no mobile charged particles available to carry an electric current, meaning solid sodium chloride does not conduct electricity. However, when sodium chloride is melted or dissolved in water, the rigid lattice structure breaks down and the Na+ and Cl- ions become free to move; these mobile, charged ions can then carry electric charge through the liquid, allowing molten or dissolved sodium chloride to conduct electricity.

(b) Within a molecule of iodine (I2), the two iodine atoms are held together by a strong covalent bond. However, between separate I2 molecules, only weak intermolecular forces, called van der Waals' forces, hold the molecules together within the solid structure. When solid iodine melts, it is only these weak forces between molecules that need to be overcome - the strong covalent bonds within each I2 molecule remain intact and are not broken. Because van der Waals' forces are much weaker than the strong ionic bonds that must be overcome throughout sodium chloride's giant lattice, far less energy is needed to melt iodine, giving it a much lower melting point than the ionic compound sodium chloride.

(c) Metals such as copper have a giant metallic structure, consisting of positively charged metal ions arranged in a regular, repeating lattice, surrounded by a 'sea' of delocalised (free-moving) electrons that are not attached to any one particular ion. Because these delocalised electrons are free to move throughout the whole structure, they can carry electric charge, which is why metals are good conductors of electricity. The metallic structure is also malleable because the layers of positive metal ions are able to slide over one another when a force is applied, without breaking the metallic bonding, since the sea of delocalised electrons continues to move with and hold together the ions in their new positions; this allows the metal to be bent, hammered or shaped without shattering.

(a) [1] giant ionic lattice held by strong electrostatic forces (ionic bonds) throughout the structure + [1] large amount of energy needed to overcome these bonds, giving a high melting point + [1] solid: ions fixed in place, cannot move, so does not conduct + [1] molten/dissolved: ions free to move and carry charge, so conducts. (b) [1] iodine molecules held by weak intermolecular (van der Waals') forces + [1] only these weak forces overcome on melting (covalent bonds within I2 remain intact) + [1] correctly links weaker forces overcome to much lower melting point than ionic NaCl. (c) [1] structure described as positive ions in a lattice surrounded by delocalised/free electrons + [1] delocalised electrons free to move and carry charge, explaining conductivity + [1] layers of ions can slide over each other (electrons continue to hold structure together), explaining malleability. Maximum [10] overall.

(a) Mr(CaCO3) = Ar(Ca) + Ar(C) + 3 × Ar(O) = 40 + 12 + (3 × 16) = 40 + 12 + 48 = 100. Mr(CaO) = Ar(Ca) + Ar(O) = 40 + 16 = 56.

(b) Moles = mass ÷ Mr. Moles of CaCO3 = 12.5 ÷ 100 = 0.125 mol.

(c) From the balanced equation CaCO3 → CaO + CO2, the ratio of CaCO3 to CaO is 1:1, so moles of CaO produced = moles of CaCO3 reacted = 0.125 mol. Mass of CaO = moles × Mr = 0.125 × 56 = 7.0 g. This is the theoretical (maximum possible) yield.

(d) Percentage yield = (actual yield ÷ theoretical yield) × 100 = (6.3 ÷ 7.0) × 100 = 90%. A percentage yield of less than 100% could be explained by loss of some of the solid product during the experiment, for example when transferring it between containers or during filtration, or because the reaction did not fully go to completion (some calcium carbonate may not have fully decomposed) within the time/temperature used.

(a) [1] Mr(CaCO3) = 100 + [1] Mr(CaO) = 56. (b) [1] correct method (mass ÷ Mr) + [1] correct answer (0.125 mol). (c) [1] correct use of 1:1 mole ratio to find moles of CaO (0.125 mol) + [1] correct final mass (7.0 g). (d) [1] correct percentage yield calculation and answer (90%) + [1] valid reason for yield below 100% (e.g. loss of product during transfer/separation, reaction incomplete). Error carried forward applies throughout if an earlier part is incorrect but the method is correctly applied. Maximum [8] overall.

A high-scoring answer covers definition/structure, physical properties with explanation, and correct classification with justification, using accurate scientific terminology throughout.

(a) An allotrope is one of two or more different structural forms in which the atoms of the same element can exist, in the same physical state, with the atoms arranged or bonded differently. Besides graphene, carbon exists as two other well-known allotropes: diamond and graphite.

(b) Graphene consists of a single layer of carbon atoms, only one atom thick, arranged in a flat, two-dimensional hexagonal (honeycomb-shaped) lattice. Each carbon atom in this layer is covalently bonded to three neighbouring carbon atoms.

(c) Graphene is extremely strong because each carbon atom is held to its three neighbours by strong covalent bonds, extending across the entire sheet in a continuous network; because so many strong covalent bonds must be broken to break the material, graphene has very high tensile strength for its weight. Graphene conducts electricity well because each carbon atom in the structure uses only three of its four outer electrons to form covalent bonds to its neighbours; the fourth electron from each carbon atom becomes delocalised (free to move) across the layer, similar to the delocalised electrons in a metal, allowing graphene to conduct electric charge efficiently. These two properties make graphene valuable in batteries and solar cells: in a battery, graphene's high electrical conductivity allows charge to be transferred rapidly and efficiently through the device, improving performance, while its strength (combined with being extremely thin and lightweight) allows components to be made more compact and durable; in solar cells, its high conductivity again allows the efficient collection and transfer of the electric charge generated when light strikes the cell.

(d) Graphene is correctly classified as a giant covalent structure. This is because it consists of an extremely large number of carbon atoms, all covalently bonded together into one continuous, extended lattice/network, rather than existing as small, discrete molecules (which would instead classify it as molecular covalent) or as charged ions in a lattice (giant ionic) or as metal ions in a sea of delocalised electrons in the traditional sense (metallic), even though its electrical conductivity is metal-like in this specific case; the defining feature - a huge, continuous network of atoms held together entirely by strong covalent bonds - places it firmly in the giant covalent category, alongside diamond and graphite.

Banded mark scheme (12 marks total).

Band C [1]-[4]: Basic response; may state graphene is a form of carbon with little or no accurate structural detail; weak or no reference to bonding when explaining properties; classification, if given, unjustified; weak specialist vocabulary; errors in spelling/grammar that hinder meaning.

Band B [5]-[8]: Reasonable description of graphene's structure (single layer, hexagonal arrangement) and at least one other allotrope named; some explanation of strength and/or conductivity with partial reference to bonding (covalent bonds/delocalised electrons); classification given, with limited justification; developing specialist vocabulary; reasonably organised.

Band A [9]-[12]: Accurate, detailed description of graphene's structure (single atom thick, hexagonal lattice, each carbon bonded to three others) with two correctly named allotropes (diamond, graphite); clear, accurate explanation of BOTH strength (strong covalent bonds throughout the lattice) and electrical conductivity (delocalised electrons from the unbonded fourth electron), explicitly linked to the named applications (batteries/solar cells); correct classification (giant covalent) with clear, accurate justification referring to the continuous covalently-bonded network; wide range of accurate specialist vocabulary; clear coherence, spelling and grammar throughout.

(a) Formula: density = mass ÷ volume. Substitution: density = 270 ÷ 100. Calculation: density = 2.7. Final answer: density = 2.7 g/cm3.

(b) In the solid state, particles are held closely together in a tightly packed, regular arrangement, with very strong forces of attraction between them and only small gaps between particles; this means a relatively large mass of particles is packed into a small volume. In the gas state, the particles have much more kinetic energy, move around freely and rapidly in all directions, and are spread far apart, with large amounts of empty space between individual particles; this means a much smaller mass of particles occupies the same volume compared with the solid. Because density is mass divided by volume, the closely packed arrangement of particles in a solid results in a much higher density than the widely spaced arrangement of particles in a gas of the same substance.

(a) [1] correct formula (density = mass ÷ volume) + [1] correct substitution (270 ÷ 100) + [1] correct answer with unit (2.7 g/cm3). (b) [1] solid particles closely packed/regular arrangement, large mass in small volume + [1] gas particles spread far apart with large spaces, much smaller mass in the same volume, giving lower density. Maximum [5] overall.

(a) Formula: W = m × g. Substitution: W = 75 × 10. Calculation: W = 750. Final answer: weight = 750 N.

(b) Mass is defined as the amount of matter contained in an object; it is measured in kilograms (kg) and stays the same regardless of where the object is located (for example, on Earth or on the Moon). Weight, in contrast, is a force caused by the pull of gravity acting on an object's mass; it is measured in newtons (N) and depends on the strength of the gravitational field the object is in, meaning the same object would have a different weight on the Moon (where gravity is weaker) than on Earth, even though its mass would remain unchanged.

(c) The upward force opposing the skydiver's downward motion through the air is air resistance (also known as drag).

(d) As the skydiver falls and their speed increases, the air resistance acting on them (which increases with speed) also increases. Eventually, the air resistance acting upwards on the skydiver becomes exactly equal in size to the skydiver's weight acting downwards. At this point, the forces acting on the skydiver are balanced, meaning the resultant force on them is zero; with no resultant force, the skydiver stops accelerating and instead falls at a constant, steady speed (called terminal velocity).

(a) [1] correct substitution into W = mg (75 × 10) + [1] correct answer with unit (750 N). (b) [1] mass correctly defined (amount of matter, kg, does not change with location) + [1] weight correctly defined (force due to gravity, N, depends on gravitational field strength). (c) [1] air resistance/drag. (d) [1] air resistance increases as speed increases + [1] correctly explains that when air resistance equals weight, forces are balanced/resultant force is zero, so speed becomes constant (no further acceleration). Maximum [7] overall.

(a) Formula: resultant force = mass × acceleration (F = m × a), rearranged to a = F ÷ m. Substitution: a = 800 ÷ 400. Calculation: a = 2. Final answer: acceleration = 2 m/s2.

(b) Newton's first law states that, in the absence of unbalanced (resultant) forces, an object will continue to move in a straight line at a constant speed (and an object at rest will remain at rest).

(a) [1] correct rearrangement of F = ma to find a + [1] correct substitution (800 ÷ 400) + [1] correct final answer with unit (2 m/s2). (b) [1] correct statement of Newton's first law (no unbalanced/resultant force means constant velocity/straight line at constant speed, or remains at rest). Maximum [4] overall.

A high-scoring answer accurately describes the composition of all three types of radiation and clearly links their differing penetrating power to appropriate precautions.

Composition: an alpha particle consists of two protons and two neutrons bound together, which is identical to the nucleus of a helium atom. A beta particle is a fast-moving electron, emitted from the nucleus when a neutron changes into a proton. Gamma radiation is not a particle at all, but a high-energy electromagnetic wave (with no mass and no charge), emitted from an unstable nucleus.

Penetrating power and precautions: alpha particles are relatively large and heavy compared with beta particles and gamma rays, and so have the lowest penetrating power - they are stopped by just a few centimetres of air, or by a sheet of paper or the outer layer of skin, meaning an alpha source outside the body is not very dangerous. However, if an alpha-emitting source is swallowed or inhaled, it can cause serious damage to cells inside the body, since all of its energy is deposited over a very short range within living tissue; precautions should therefore focus strongly on preventing ingestion or inhalation. Beta particles are much smaller and faster than alpha particles, giving them moderate penetrating power - they can pass through paper and skin but are stopped by a few millimetres of aluminium; because they can penetrate the skin and damage cells, precautions such as using tongs to increase distance from the source, wearing protective clothing, and minimising the time of exposure are important even when the source is outside the body. Gamma radiation, being a high-energy electromagnetic wave with no mass, has the highest penetrating power of the three, and is only significantly reduced in intensity by thick, dense shielding such as several centimetres of lead or a substantial thickness of concrete; because it can penetrate deep into the body and cause serious cell damage, gamma sources require the most rigorous precautions, including thick shielding, remote handling (tongs), keeping as much distance as possible from the source, and minimising exposure time.

Banded mark scheme (6 marks total).

Band C [1]-[2]: Basic, limited response; may name the three types of radiation with little or no accurate detail on composition; little or no reference to penetrating power or precautions; weak specialist vocabulary; errors in spelling/grammar that hinder meaning.

Band B [3]-[4]: Reasonable description of the composition of at least two of the three radiation types (e.g. alpha as a helium nucleus/2p+2n, beta as an electron); some correct reference to differing penetrating power and at least one linked precaution; developing specialist vocabulary; reasonably organised.

Band A [5]-[6]: Accurate, complete description of the composition of all three radiation types (alpha = 2 protons + 2 neutrons/helium nucleus; beta = fast electron; gamma = high-energy electromagnetic wave); clear, accurate explanation of the penetrating power of each, correctly linked to specific, appropriate precautions for each type (including the distinction between alpha being most hazardous inside the body versus beta/gamma requiring shielding/distance/time precautions even outside the body); wide range of accurate specialist vocabulary; clear coherence, spelling and grammar throughout.

(a) The half-life of a radioactive source is the time it takes for the count rate (activity) of the source, or the number of undecayed radioactive nuclei remaining, to fall to half of its original value.

(b) Background radiation is radiation that is present in the environment all the time, from sources such as cosmic rays from space and naturally occurring radioactive materials in rocks, soil and building materials. If this background radiation is not subtracted from the measured count rate, the reading would be higher than the true activity due to the radioactive source alone, so the background count rate must be subtracted to obtain the corrected count rate that is due only to the source itself.

(c) 15 days is equal to 15 ÷ 5 = 3 half-lives. Each half-life, the count rate halves: after 1 half-life (5 days): 800 ÷ 2 = 400 counts per minute. After 2 half-lives (10 days): 400 ÷ 2 = 200 counts per minute. After 3 half-lives (15 days): 200 ÷ 2 = 100 counts per minute. Final answer: after 15 days, the count rate would be 100 counts per minute.

(a) [1] time taken for count rate/activity to fall + [1] to half of its original/previous value. (b) [1] correctly explains background radiation would otherwise be included, giving an inaccurate reading of the source's own activity. (c) [1] correctly identifies 15 days = 3 half-lives + [1] correct method (halving repeatedly: 800→400→200→100) + [1] correct final answer (100 counts per minute). Maximum [6] overall.

(a) The velocity-time graph would consist of three straight-line sections. From 0 to 4 seconds, the line slopes upwards from the origin (0 m/s at t=0) to a velocity of 12 m/s at t=4s, showing uniform (constant) acceleration. From 4 to 10 seconds, the line is horizontal at a velocity of 12 m/s, showing the cyclist travelling at a constant velocity (zero acceleration). From 10 to 14 seconds, the line slopes downwards from 12 m/s back down to 0 m/s at t=14s, showing uniform deceleration back to rest.

(b) Formula: acceleration = change in velocity ÷ time taken. Substitution: a = (12 - 0) ÷ 4. Calculation: a = 12 ÷ 4 = 3. Final answer: acceleration = 3 m/s2.

(c) The distance travelled is equal to the total area under the velocity-time graph, which can be split into three sections. Section 1 (0-4s): a triangle with base 4 s and height 12 m/s; area = 0.5 × 4 × 12 = 24 m. Section 2 (4-10s): a rectangle with base 6 s and height 12 m/s; area = 6 × 12 = 72 m. Section 3 (10-14s): a triangle with base 4 s and height 12 m/s; area = 0.5 × 4 × 12 = 24 m. Total distance = 24 + 72 + 24 = 120 m. Final answer: the cyclist travels a total distance of 120 m.

(a) [1] correct description of stage 1 (straight line sloping up from origin to (4,12), uniform acceleration) + [1] correct description of stage 2 (horizontal line at 12 m/s from 4s to 10s, constant velocity) + [1] correct description of stage 3 (straight line sloping down from (10,12) to (14,0), uniform deceleration). (b) [1] correct substitution (12 ÷ 4) + [1] correct answer with unit (3 m/s2). (c) [1] correct method (splitting into triangle + rectangle + triangle, or trapezium, and using area = distance) + [1] correct working showing each area (24 + 72 + 24) + [1] correct final total (120 m). Maximum [8] overall.

(a) Ep = m × g × h.

(b) Substitution: Ep = 8 × 10 × 5. Calculation: Ep = 400. Final answer: gravitational potential energy gained = 400 J.

(c) Formula: work done = force × distance moved in the direction of the force (W = F × d). Substitution: W = 50 × 10. Calculation: W = 500. Final answer: work done by the worker = 500 J.

(d) Comparing the two results: the worker does 500 J of work pushing the crate up the ramp, but the crate only gains 400 J of gravitational potential energy. The 'missing' 100 J of energy (500 - 400 = 100 J) is not lost overall (energy is always conserved), but it is not stored usefully as gravitational potential energy in the crate; instead, this energy is transferred to overcome friction between the crate and the surface of the ramp as it slides/is pushed along it, and is dissipated as heat (and possibly a small amount of sound), warming the ramp surface and the crate slightly rather than lifting the crate any higher.

(a) [1] Ep = mgh correctly stated. (b) [1] correct substitution (8 × 10 × 5) + [1] correct calculation + [1] correct final answer with unit (400 J). (c) [1] correct formula (W = F × d) + [1] correct substitution (50 × 10) + [1] correct final answer with unit (500 J). (d) [1] correctly identifies friction between the crate and the ramp as the cause + [1] correctly explains the energy is dissipated/transferred as heat (accept sound) rather than being stored as Ep. Maximum [9] overall.

(a) The mass number (A) represents the total number of protons and neutrons (collectively called nucleons) present in the nucleus of an atom. The atomic number (Z) represents the number of protons present in the nucleus, which determines what element the atom is.

(b) For this uranium isotope, A = 238 and Z = 92. Number of protons = Z = 92. Number of neutrons = A - Z = 238 - 92 = 146. Because the atom is neutral (uncharged), the number of electrons must equal the number of protons, so number of electrons = 92.

(c) During alpha decay, the unstable nucleus emits an alpha particle, which consists of 2 protons and 2 neutrons (equivalent to a helium nucleus, mass number 4, atomic number 2). To balance the nuclear equation, the mass numbers on each side must be equal, and the atomic numbers on each side must be equal. Mass number of the new nucleus (thorium) = 238 - 4 = 234. Atomic number of the new nucleus (thorium) = 92 - 2 = 90. This confirms the new nucleus is an isotope of thorium (Th, atomic number 90) with mass number 234. The balanced nuclear equation is: Uranium-238 (mass 238, atomic number 92) → Thorium-234 (mass 234, atomic number 90) + Helium/alpha particle (mass 4, atomic number 2). Check: mass numbers balance (238 = 234 + 4); atomic numbers balance (92 = 90 + 2).

(a) [1] mass number = total protons + neutrons in the nucleus + [1] atomic number = number of protons in the nucleus. (b) [1] protons = 92 + [1] neutrons = 146 + [1] electrons = 92. (c) [1] correct mass number of thorium product (234) + [1] correct atomic number of thorium product (90) + [1] fully balanced equation shown with alpha particle correctly represented (mass 4, atomic number 2) and mass/atomic numbers checked to balance on both sides. Maximum [8] overall.

(a) For nuclear fission to occur, a large, unstable nucleus (such as uranium) must first absorb an extra neutron. Once it has absorbed this neutron, the nucleus becomes even more unstable and splits apart into two smaller nuclei, releasing a large amount of energy in the process, along with several additional neutrons. These newly released neutrons can then travel outwards and be absorbed by other nearby uranium nuclei, causing those nuclei to also undergo fission, which releases further neutrons still. If enough of these released neutrons go on to cause further fissions in this way, the number of fission reactions occurring rapidly multiplies, and this self-sustaining, multiplying process is called a chain reaction.

(b) Nuclear fusion is the process in which two small, light atomic nuclei (for example, isotopes of hydrogen) join together (fuse) to form a single, larger nucleus; this process releases a very large amount of energy. Nuclear fusion is the process that powers stars, including our own Sun, providing the huge amounts of energy that stars radiate as light and heat.

(c) One advantage of fusion over fission relates to the raw materials required: fusion uses isotopes of hydrogen, deuterium and tritium, which are present as constituents of seawater and are therefore extremely abundant and considered nearly inexhaustible, whereas fission relies on uranium ore, a finite resource that will eventually run out. A further advantage relates to the products formed: nuclear fusion's main by-product is helium, which is an inert, non-toxic gas that presents no significant environmental or radioactive waste disposal problem, in contrast to nuclear fission, which produces radioactive waste products that remain hazardous and must be safely stored for a very long time.

(a) [1] uranium nucleus must first absorb a neutron + [1] nucleus splits into two smaller nuclei, releasing energy and further neutrons + [1] these released neutrons go on to be absorbed by/cause fission in other uranium nuclei + [1] correctly identifies this multiplying/self-sustaining process as a chain reaction. (b) [1] two small/light nuclei join/fuse together to form a larger nucleus, releasing energy + [1] correctly identifies stars/the Sun as the natural source of fusion energy. (c) [1]-[2] any one valid, developed advantage, e.g. hydrogen isotopes (deuterium/tritium) are abundant/nearly inexhaustible (from seawater) unlike finite uranium ore; OR fusion's main by-product (helium) is inert/non-toxic, unlike fission's radioactive waste. Maximum [8] overall.

(a) The moment of a force is a measure of the turning effect that a force has about a pivot (fixed point). It is calculated using the equation: moment = force × perpendicular distance from the pivot (moment = F × d).

(b) The Principle of Moments states that when an object is balanced (in equilibrium), the total (sum) of the clockwise moments acting about the pivot is equal to the total (sum) of the anticlockwise moments acting about the same pivot.

(c) The known 30 N weight, 60 cm from the pivot, creates a moment on one side: moment = 30 × 60 = 1800 N cm. Since the rule is balanced, by the Principle of Moments, the moment created by the unknown weight on the opposite side must also equal 1800 N cm. The unknown weight (F2) acts at a distance of 40 cm from the pivot, so: F2 × 40 = 1800. Rearranging: F2 = 1800 ÷ 40 = 45. Final answer: the second weight = 45 N.

(d) The centre of gravity of an object is defined as the single point within (or associated with) the object at which the entire weight of the object can be considered to act.

(e) An object topples over when its centre of gravity moves beyond the edge of its base, so that the line of action of its weight no longer passes through the base. An object with a wide base and a low centre of gravity must be tilted through a much greater angle before its centre of gravity passes beyond the edge of its base than an object with a narrow base and a high centre of gravity; this means a wide-based, low centre of gravity object is more difficult to tip over, and is therefore more stable.

(a) [1] moment correctly defined as the turning effect of a force + [1] correct equation (moment = force × perpendicular distance from pivot). (b) [1]-[2] correct statement of the Principle of Moments (sum of clockwise moments = sum of anticlockwise moments, for an object in equilibrium). (c) [1] correct calculation of the known moment (30 × 60 = 1800 N cm) + [1] correct rearrangement/method to find the unknown force (1800 ÷ 40) + [1] correct final answer with unit (45 N). (d) [1] correctly defines centre of gravity as the point where the whole weight can be considered to act. (e) [1] correctly explains that a wider base/lower centre of gravity requires a greater tilt before the centre of gravity passes beyond the base, making the object harder to topple/more stable. Maximum [9] overall.