ESAT 2021: 84 questions decoded out of 90, by method and by chapter.

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The paper at a glance, coloured by move. Click a square to open its question; empty squares are not decoded yet.

• 61Natural selection edits pre-existing random variation, the environment never writes itKey methodBiologyInheritance, Variation & Evolution4 other sittings

  Same method, other sittings

  2016201820202022

  Worked solutionOpen the method
• 62Prokaryote versus eukaryote: the defining feature contrast (wall, nucleus, mitochondria, chloroplasts)Animal cells never have a cell wall or chloroplasts: use it as an instant eliminatorPavlovBiologyCells6 other sittings

  Same method, other sittings

  201620172018201920202023

  Worked solutionOpen the method

  AlsoAnimal cells never have a cell wall or chloroplasts: use it as an instant eliminator
• 63The three osmosis definition traps examiners plant in statementsMovement against the concentration gradient means active transport and needs energyKey methodBiologyMembrane Transport, Cell Division & Reproduction4 other sittings

  Same method, other sittings

  2018202020222023

  Worked solutionOpen the method

  AlsoMovement against the concentration gradient means active transport and needs energy
• 64Blood glucose control: insulin stores, glucagon and adrenaline release, the loop is negative feedbackPavlovBiologyAnimal Physiology: Systems, Homeostasis & Disease1 other sitting

  Same method, other sittings

  2022

  Worked solutionOpen the method
• 65Separate chromosome, DNA molecule and strand, and watch for the replication doublingEvery body cell shares one genome, and the male gamete fixes the sexMutation is the exception that breaks any absolute genetic claimKey methodBiologyMembrane Transport, Cell Division & Reproduction5 other sittings

  Same method, other sittings

  20162017201820192022

  Worked solutionOpen the method

  AlsoEvery body cell shares one genome, and the male gamete fixes the sexMutation is the exception that breaks any absolute genetic claim
• 66Weigh the modal word: always or must needs certainty, could or may needs only one plausible mechanismKey methodBiologyEcology & Plant Physiology3 other sittings

  Same method, other sittings

  201720202022

  Worked solutionOpen the method
• 67A rise-then-plateau curve means a factor is now limiting; plateau height is set by total substrateBiology arithmetic reflexes: percentage over the original, rate as amount over time, area as radius squaredAn enzyme is a protein catalyst, not a living thing: eliminate options that contradict its natureKey methodBiologyEnzymes5 other sittings

  Same method, other sittings

  20162019202020222023

  Worked solutionOpen the method

  AlsoBiology arithmetic reflexes: percentage over the original, rate as amount over time, area as radius squaredAn enzyme is a protein catalyst, not a living thing: eliminate options that contradict its nature
• 68On numbered-statement items, bin the clause that reverses cause, sign or proportionalityRespiration signature in blood; hard exercise adds lactate and lowers pHKey methodBiologyAnimal Physiology: Systems, Homeostasis & Disease3 other sittings

  Same method, other sittings

  201620222023

  Worked solutionOpen the method

  AlsoRespiration signature in blood; hard exercise adds lactate and lowers pH
• 69Probability of a specific sex and a specific genotype: multiply the two independent chancesA lethal or non-surviving genotype: delete it and re-read the ratio among survivorsA recessive phenotype fixes the genotype, a dominant phenotype needs a test crossKey methodBiologyInheritance, Variation & Evolution6 other sittings

  Same method, other sittings

  201620172018202020222023

  Worked solutionOpen the method

  AlsoA lethal or non-surviving genotype: delete it and re-read the ratio among survivorsA recessive phenotype fixes the genotype, a dominant phenotype needs a test cross
• 70DNA base arithmetic: pairing rules bind the whole molecule, never a single strandKey methodBiologyDNA, Protein Synthesis & Gene Technology3 other sittings

  Same method, other sittings

  201620182020

  Worked solutionOpen the method
• 71Osmosis moves water toward the more concentrated solutionThe cell wall decides whether a cell bursts, and the vacuole holds cell sapPavlovBiologyMembrane Transport, Cell Division & Reproduction6 other sittings

  Same method, other sittings

  201620172018202020222023

  Worked solutionOpen the method

  AlsoThe cell wall decides whether a cell bursts, and the vacuole holds cell sap
• 72Carbon cycle diagrams: follow the arrows and check each product for carbon and hydrogenKey methodBiologyEcology & Plant Physiology2 other sittings

  Same method, other sittings

  20202023

  Worked solutionOpen the method
• 73Magnification equals image size over actual size: fix the units first, then let the order of magnitude chooseKey methodBiologyCells1 other sitting

  Same method, other sittings

  2023

  Worked solutionOpen the method
• 74Monohybrid cross Aa x Aa gives a 1:2:1 genotype ratio and a 3:1 phenotype ratioProbability of a specific sex and a specific genotype: multiply the two independent chancesRead dominance off the pedigree first, then fix which parents must be heterozygousTunnelBiologyInheritance, Variation & Evolution6 other sittings

  Same method, other sittings

  201620172018202020222023

  Worked solutionOpen the method

  AlsoProbability of a specific sex and a specific genotype: multiply the two independent chancesRead dominance off the pedigree first, then fix which parents must be heterozygous
• 75Read a stem cell division by counting what the two daughters areKey methodBiologyMembrane Transport, Cell Division & Reproduction1 other sitting

  Same method, other sittings

  2017

  Worked solutionOpen the method
• 76Temperature and pH curves peak at an optimum; a concentration curve plateausKey methodBiologyEnzymes2 other sittings

  Same method, other sittings

  20202022

  Worked solutionOpen the method
• 77Match each defence to its signature; antibiotics hit bacteria only, red cells are not phagocyticPavlovBiologyAnimal Physiology: Systems, Homeostasis & Disease1 other sitting

  Same method, other sittings

  2020

  Worked solutionOpen the method
• 78Mitosis keeps the chromosome number and makes two cells, meiosis halves it and makes fourKey methodBiologyMembrane Transport, Cell Division & Reproduction6 other sittings

  Same method, other sittings

  201620172018202020222023

  Worked solutionOpen the method
• 79One person, one genome: division type, stem cells, and same DNA but different proteinsPavlovBiologyCells2 other sittings

  Same method, other sittings

  20182019

  Worked solutionOpen the method
• 80Ecological abundance: frequency is presence in quadrats, density is count per area, and both need a random sampleKey methodBiologyEcology & Plant Physiology5 other sittings

  Same method, other sittings

  20172018201920202022

  Worked solutionOpen the method

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The paper at a glance, coloured by move. Click a square to open its question; empty squares are not decoded yet.

• 41Read the reactivity order off the behaviour, do not recall a fixed listKey methodChemistryElectrolysis & Metals2 other sittings

  Same method, other sittings

  20192023

  Worked solutionOpen the method
• 42Read a nuclide symbol into protons, neutrons and electronsIsoelectronic species share an electron count but differ in protonsPavlovChemistryAtomic Structure, Bonding & the Periodic Table5 other sittings

  Same method, other sittings

  20162017201820192023

  Worked solutionOpen the method

  AlsoIsoelectronic species share an electron count but differ in protons
• 43Relative atomic mass as an abundance-weighted mean, forwards or reversedTunnelChemistryAtomic Structure, Bonding & the Periodic Table5 other sittings

  Same method, other sittings

  20172018202020222023

  Worked solutionOpen the method
• 44Classify the structure and bonding, then read off the propertyRecall Group 1 and Group 17 trends whole, without deriving themKey methodChemistryAtomic Structure, Bonding & the Periodic Table7 other sittings

  Same method, other sittings

  2016201720182019202020222023

  Worked solutionOpen the method

  AlsoRecall Group 1 and Group 17 trends whole, without deriving them
• 45Thermometric graph: peak is neutralisation, the fall is only coolingKey methodChemistryRates of Reaction & Energetics1 other sitting

  Same method, other sittings

  2020

  Worked solutionOpen the method
• 46Addition-polymer repeat unit: two backbone carbons, side groups keptKey methodChemistryOrganic Chemistry, Separation & Analysis2 other sittings

  Same method, other sittings

  20222023

  Worked solutionOpen the method
• 47Standard qualitative tests to identify ionsPavlovChemistryOrganic Chemistry, Separation & Analysis2 other sittings

  Same method, other sittings

  20192020

  Worked solutionOpen the method
• 48Oxidation states sum to the overall charge, capped by the group numberKey methodChemistryChemical Reactions, Acids, Bases & Redox2 other sittings

  Same method, other sittings

  20202023

  Worked solutionOpen the method
• 49Classic organic-product distractors the examiner offersEsterification balances by conservation of mass, add 18 for waterKey methodChemistryOrganic Chemistry, Separation & Analysis2 other sittings

  Same method, other sittings

  20192022

  Worked solutionOpen the method

  AlsoEsterification balances by conservation of mass, add 18 for water
• 50Balance a skeleton equation one element at a time, most constrained firstTunnelChemistryChemical Reactions, Acids, Bases & Redox3 other sittings

  Same method, other sittings

  201820192020

  Worked solutionOpen the method
• 51Rf equals spot distance over solvent-front distanceInterpreting a chromatogram: purity, identity, mobility and phasesPavlovChemistryOrganic Chemistry, Separation & Analysis6 other sittings

  Same method, other sittings

  201620172018201920202022

  Worked solutionOpen the method

  AlsoInterpreting a chromatogram: purity, identity, mobility and phases
• 52Name an unknown product from the atoms left over after the known productsKey methodChemistryChemical Reactions, Acids, Bases & Redox2 other sittings

  Same method, other sittings

  20162022

  Worked solutionOpen the method
• 53Diprotic acid reflex: sulfuric and ethanedioic acid react two to one with a monobasic basePavlovChemistryQuantitative Chemistry2 other sittings

  Same method, other sittings

  20162020

  Worked solutionOpen the method
• 54Calorimetry: q equals mc delta T, then divide by the limiting molesTunnelChemistryRates of Reaction & Energetics3 other sittings

  Same method, other sittings

  202020222023

  Worked solutionOpen the method
• 55Bond-energy balance: broken minus formed, counting every bondExploit the bond-energy option list: sign, factor and phantom bondsTunnelChemistryRates of Reaction & Energetics4 other sittings

  Same method, other sittings

  2016201720192023

  Worked solutionOpen the method

  AlsoExploit the bond-energy option list: sign, factor and phantom bonds
• 56Read the balanced equation directly in gas volumes, and track the change in the number of gas moleculesLimiting reagent by moles over coefficient, and the graph plateau marks the exact stoichiometric pointKey methodChemistryQuantitative Chemistry3 other sittings

  Same method, other sittings

  201720182020

  Worked solutionOpen the method

  AlsoLimiting reagent by moles over coefficient, and the graph plateau marks the exact stoichiometric point
• 57Electron accounting: turn charge into moles, then read every amount off the half-equationsTunnelChemistryElectrolysis & Metals4 other sittings

  Same method, other sittings

  2016201720192020

  Worked solutionOpen the method
• 58Mass by difference: the component asked for is the leftover, and its molar mass is often a decoyMolar gas volume both ways, and the two traps: cubic decimetres versus cubic centimetres, and diatomic gasesKey methodChemistryQuantitative Chemistry3 other sittings

  Same method, other sittings

  201620202023

  Worked solutionOpen the method

  AlsoMolar gas volume both ways, and the two traps: cubic decimetres versus cubic centimetres, and diatomic gases
• 59Mass by difference: the component asked for is the leftover, and its molar mass is often a decoyPin exactly what is asked: the examiner plants the natural intermediate value as an optionPrecipitation gravimetry: only the ion that forms the solid counts, ignore the spectatorsKey methodChemistryQuantitative Chemistry5 other sittings

  Same method, other sittings

  20162018201920202022

  Worked solutionOpen the method

  AlsoPin exactly what is asked: the examiner plants the natural intermediate value as an optionPrecipitation gravimetry: only the ion that forms the solid counts, ignore the spectators
• 60Cumulative product only rises, and each interval adds lessKey methodChemistryRates of Reaction & Energeticsonly this sitting

  Same method, other sittings

  This one has only struck here so far.

  Worked solutionOpen the method

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The paper at a glance, coloured by move. Click a square to open its question; empty squares are not decoded yet.

• 1Distributing Outer PowersPavlovPureAlgebra, Functions & Graphs1 other sitting

  Same method, other sittings

  2022

  Worked solutionOpen the method
• 2Algebraic Fraction SimplificationPavlovPureAlgebra, Functions & Graphs6 other sittings

  Same method, other sittings

  201620172019202020222023

  Worked solutionOpen the method
• 3Rearranging Complex FormulasPavlovPureAlgebra, Functions & Graphs4 other sittings

  Same method, other sittings

  2016201920222023

  Worked solutionOpen the method
• 4Perpendicular lines: the product of their gradients is -1PavlovPureCoordinate Geometry2 other sittings

  Same method, other sittings

  20162017

  Worked solutionOpen the method
• 53D solids: equating volumes and surface areas algebraicallyTunnelPureGeometry5 other sittings

  Same method, other sittings

  20182019202020222023

  Worked solutionOpen the method
• 6Treat complex terms as single variables in simultaneous equationsPavlovPureNumber & Unitsonly this sitting

  Same method, other sittings

  This one has only struck here so far.

  Worked solutionOpen the method
• 7Successive percentage changes as a product of decimal multipliersPavlovPureRatio & Proportion5 other sittings

  Same method, other sittings

  20162018201920202022

  Worked solutionOpen the method
• 9Overlay a coordinate system for complex intersecting shapesKey methodPureGeometry5 other sittings

  Same method, other sittings

  20172019202020222023

  Worked solutionOpen the method
• 11Root Gap from the DiscriminantKey methodPureAlgebra, Functions & Graphs2 other sittings

  Same method, other sittings

  20202022

  Worked solutionOpen the method
• 12Systematic enumeration for small discrete sumsPavlovProb & StatsProbability & Statistics2 other sittings

  Same method, other sittings

  20202022

  Worked solutionOpen the method
• 14Circle theorems: angle chasingTunnelPureGeometry2 other sittings

  Same method, other sittings

  20182023

  Worked solutionOpen the method
• 15Set up algebraic probability treesTunnelProb & StatsProbability & Statistics4 other sittings

  Same method, other sittings

  2016201720192023

  Worked solutionOpen the method
• 19Sanity check algebraic word problems by scalingKey methodPureNumber & Unitsonly this sitting

  Same method, other sittings

  This one has only struck here so far.

  Worked solutionOpen the method
• 20Sketching and Counting IntersectionsKey methodPureTrigonometryonly this sitting

  Same method, other sittings

  This one has only struck here so far.

  Worked solutionOpen the method

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The paper at a glance, coloured by move. Click a square to open its question; empty squares are not decoded yet.

• 21Rewriting Before DifferentiatingPavlovPureDifferentiation1 other sitting

  Same method, other sittings

  2023

  Worked solutionOpen the method
• 23The Difference of Partial SumsPavlovPureSequences & Seriesonly this sitting

  Same method, other sittings

  This one has only struck here so far.

  Worked solutionOpen the method
• 25Quadratics in DisguiseKey methodPureAlgebra, Functions & Graphs3 other sittings

  Same method, other sittings

  201620202022

  Worked solutionOpen the method
• 27Sector and Segment MechanicsTunnelPureTrigonometry2 other sittings

  Same method, other sittings

  20172018

  Worked solutionOpen the method
• 29Sum of Logarithms → Product of ArgumentsPavlovPureExponentials & Logarithms1 other sitting

  Same method, other sittings

  2018

  Worked solutionOpen the method
• 31Ordering Trig Values without a CalculatorKey methodPureTrigonometryonly this sitting

  Same method, other sittings

  This one has only struck here so far.

  Worked solutionOpen the method
• 33Sub-Progressions of a Geometric SeriesKey methodPureSequences & Seriesonly this sitting

  Same method, other sittings

  This one has only struck here so far.

  Worked solutionOpen the method
• 35System to Single QuadraticPavlovPureAlgebra, Functions & Graphs2 other sittings

  Same method, other sittings

  20162018

  Worked solutionOpen the method
• 37Rationalise the denominator using the conjugateTunnelPureNumber & Units2 other sittings

  Same method, other sittings

  20172019

  Worked solutionOpen the method
• 39Geometric OptimisationTunnelPureDifferentiation2 other sittings

  Same method, other sittings

  20162018

  Worked solutionOpen the method

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The paper at a glance, coloured by move. Click a square to open its question; empty squares are not decoded yet.

• 21Find the current first: it is the hub of every electrical chainPavlovPhysicsElectricity & Magnetism4 other sittings

  Same method, other sittings

  2016201720222023

  Worked solutionOpen the method
• 22When only a ratio is asked, form the ratio and let the unknowns cancelDecide the direction of the change first, then read the spread of the optionsSealed gas at constant temperature: density follows the pressure directlyKey methodPhysicsMatter4 other sittings

  Same method, other sittings

  2016201720202022

  Worked solutionOpen the method

  AlsoDecide the direction of the change first, then read the spread of the optionsSealed gas at constant temperature: density follows the pressure directly
• 23Transformers: voltage by turns ratio, current by power, never turns ratio for a non ideal currentTunnelPhysicsElectricity & Magnetism3 other sittings

  Same method, other sittings

  201920202023

  Worked solutionOpen the method
• 24Steady motion is a free equation, and after a perturbation only the change is unbalancedChoose the system: the whole assembly for the acceleration, one body for an internal forceKey methodPhysicsKinematics & Forces4 other sittings

  Same method, other sittings

  2016201820202023

  Worked solutionOpen the method

  AlsoChoose the system: the whole assembly for the acceleration, one body for an internal force
• 25In a vacuum every electromagnetic wave shares one speed, so frequency and wavelength are strictly inverseUse the relative speed and Doppler direction, never the still values, when source or observer movesPavlovPhysicsWaves, Sound & the Electromagnetic Spectrum3 other sittings

  Same method, other sittings

  201620192022

  Worked solutionOpen the method

  AlsoUse the relative speed and Doppler direction, never the still values, when source or observer moves
• 26Atomic bookkeeping: protons are the element, neutrons are the isotope, electrons set the chargeKey methodPhysicsRadioactivity2 other sittings

  Same method, other sittings

  20202022

  Worked solutionOpen the method
• 27Springs: the extension is not the length, and the stored energy is half of what you expectKey methodPhysicsEnergy, Work & Momentum3 other sittings

  Same method, other sittings

  202020222023

  Worked solutionOpen the method
• 28In a two state question, lock the invariant from state one before touching state twoWhen the resistances are identical or never given, call each one R and let it cancelA closed ideal switch is a plain wire: redraw before you computeKey methodPhysicsElectricity & Magnetism6 other sittings

  Same method, other sittings

  201620182019202020222023

  Worked solutionOpen the method

  AlsoWhen the resistances are identical or never given, call each one R and let it cancelA closed ideal switch is a plain wire: redraw before you compute
• 29Force from a momentum change: divide by the contact time, and double the speed for a bounceThe second law gives the resultant force, but the question asks for a named oneTunnelPhysicsEnergy, Work & Momentum2 other sittings

  Same method, other sittings

  20192022

  Worked solutionOpen the method

  AlsoThe second law gives the resultant force, but the question asks for a named one
• 30Use the relative speed and Doppler direction, never the still values, when source or observer movesKey methodPhysicsWaves, Sound & the Electromagnetic Spectrum1 other sitting

  Same method, other sittings

  2022

  Worked solutionOpen the method
• 31For a non ohmic component the graph is its Ohm law: the circuit picks the axisWhich graph questions: one numerical anchor plus one shape argumentKey methodPhysicsElectricity & Magnetism4 other sittings

  Same method, other sittings

  2018201920202022

  Worked solutionOpen the method

  AlsoWhich graph questions: one numerical anchor plus one shape argument
• 32Force on a current: the work is deciding what B, I and L really areKey methodPhysicsElectricity & Magnetism4 other sittings

  Same method, other sittings

  2019202020222023

  Worked solutionOpen the method
• 33Classify a wave on the spectrum by converting band edges with the wave equationTunnelPhysicsWaves, Sound & the Electromagnetic Spectrum3 other sittings

  Same method, other sittings

  201720182020

  Worked solutionOpen the method
• 34Pick the SUVAT equation that skips the variable you were not asked aboutTunnelPhysicsKinematics & Forces1 other sitting

  Same method, other sittings

  2023

  Worked solutionOpen the method
• 35Energy audit on a heated body: supplied, stored, lostWhich expression questions: settle them by units and by one variable at a timeTunnelPhysicsThermal Physics3 other sittings

  Same method, other sittings

  201820192020

  Worked solutionOpen the method

  AlsoWhich expression questions: settle them by units and by one variable at a time
• 36Energy lost, wasted or done against drag is the input total minus the useful totalKey methodPhysicsEnergy, Work & Momentum3 other sittings

  Same method, other sittings

  201720202022

  Worked solutionOpen the method
• 37Half-life arithmetic: work in powers of one-half, never in logarithmsDecay conserves atoms one for one: daughter equals initial daughter plus parent decayedPavlovPhysicsRadioactivity6 other sittings

  Same method, other sittings

  201620172019202020222023

  Worked solutionOpen the method

  AlsoDecay conserves atoms one for one: daughter equals initial daughter plus parent decayed
• 38Ice and water mixtures: test the melting budget before writing any equationMethod of mixtures: heat lost equals heat gained, and some given data is meant to be unusedKey methodPhysicsThermal Physics2 other sittings

  Same method, other sittings

  20222023

  Worked solutionOpen the method

  AlsoMethod of mixtures: heat lost equals heat gained, and some given data is meant to be unused
• 39Let the option list choose your units, and kill options by dimensionsDecide the direction of the change first, then read the spread of the optionsLiquid pressure counts only what lies above the point, layer by layerKey methodPhysicsMatter5 other sittings

  Same method, other sittings

  20162017201920222023

  Worked solutionOpen the method

  AlsoDecide the direction of the change first, then read the spread of the optionsLiquid pressure counts only what lies above the point, layer by layer
• 40Turn measured times into distances with the right geometric factor for reflected or multi-speed signalsKey methodPhysicsWaves, Sound & the Electromagnetic Spectrum3 other sittings

  Same method, other sittings

  201620172022

  Worked solutionOpen the method