ESAT 2018: 82 questions decoded out of 85, 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.

• 55Oxygen sag below organic pollution: bacteria strip the oxygen, and indicator species track the curveKey methodBiologyEcology & Plant Physiology3 other sittings

  Same method, other sittings

  201620172020

  Worked solutionOpen the method
• 56Heterozygote advantage keeps a harmful recessive allele in a population under pressureKey methodBiologyInheritance, Variation & Evolution1 other sitting

  Same method, other sittings

  2020

  Worked solutionOpen the method
• 57Animal cells never have a cell wall or chloroplasts: use it as an instant eliminatorStructure implies function: name the cell or tissue from what it is shown doing, then read off its propertiesThe biological organisation ladder: check every noun sits at the level (cell, tissue, organ) the option claimsOne person, one genome: division type, stem cells, and same DNA but different proteinsPavlovBiologyCells4 other sittings

  Same method, other sittings

  2017201920212022

  Worked solutionOpen the method

  AlsoStructure implies function: name the cell or tissue from what it is shown doing, then read off its propertiesThe biological organisation ladder: check every noun sits at the level (cell, tissue, organ) the option claimsOne person, one genome: division type, stem cells, and same DNA but different proteins
• 58Ecological 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

  20172019202020212022

  Worked solutionOpen the method
• 59Probability of a specific sex and a specific genotype: multiply the two independent chancesKey methodBiologyInheritance, Variation & Evolution3 other sittings

  Same method, other sittings

  201620172021

  Worked solutionOpen the method
• 60Mitosis 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

  201620172020202120222023

  Worked solutionOpen the method
• 61Trace the auxin: shoots bend to light, roots to gravity, driven by uneven hormone from an intact tipPavlovBiologyEcology & Plant Physiology2 other sittings

  Same method, other sittings

  20162017

  Worked solutionOpen the method
• 62Osmosis moves water toward the more concentrated solutionThe three osmosis definition traps examiners plant in statementsRate of diffusion or osmosis is set by the steepness of the gradientPavlovBiologyMembrane Transport, Cell Division & Reproduction7 other sittings

  Same method, other sittings

  2016201720192020202120222023

  Worked solutionOpen the method

  AlsoThe three osmosis definition traps examiners plant in statementsRate of diffusion or osmosis is set by the steepness of the gradient
• 63DNA base arithmetic: pairing rules bind the whole molecule, never a single strandKey methodBiologyDNA, Protein Synthesis & Gene Technology3 other sittings

  Same method, other sittings

  201620202021

  Worked solutionOpen the method
• 64Follow the product to fix the pH direction and read the indicatorStructure implies function, and every enzyme is specific to its substrateKey methodBiologyEnzymes2 other sittings

  Same method, other sittings

  20192023

  Worked solutionOpen the method

  AlsoStructure implies function, and every enzyme is specific to its substrate
• 65A mutation present in every cell must have arisen in a gamete or the fertilised eggKey methodBiologyInheritance, Variation & Evolution1 other sitting

  Same method, other sittings

  2019

  Worked solutionOpen the method
• 66Genetic engineering means moving or altering a gene; cloning a whole nucleus does notKey methodBiologyDNA, Protein Synthesis & Gene Technology1 other sitting

  Same method, other sittings

  2020

  Worked solutionOpen the method
• 67Plateaux and the compensation point: a level rate means another factor limits, a falling rate means denaturationKey methodBiologyEcology & Plant Physiology3 other sittings

  Same method, other sittings

  201620222023

  Worked solutionOpen the method
• 68Genetic engineering means moving or altering a gene; cloning a whole nucleus does notKey methodBiologyDNA, Protein Synthesis & Gene Technology1 other sitting

  Same method, other sittings

  2020

  Worked solutionOpen the method
• 69Read gene facts off a cross-organism sequence tableKey methodBiologyDNA, Protein Synthesis & Gene Technologyonly this sitting

  Same method, other sittings

  This one has only struck here so far.

  Worked solutionOpen the method
• 70Separate chromosome, DNA molecule and strand, and watch for the replication doublingKey methodBiologyMembrane Transport, Cell Division & Reproduction3 other sittings

  Same method, other sittings

  201620212022

  Worked solutionOpen the method
• 71Natural selection edits pre-existing random variation, the environment never writes itKey methodBiologyInheritance, Variation & Evolution4 other sittings

  Same method, other sittings

  2016202020212022

  Worked solutionOpen the method
• 72Gene, allele and base sequence are three different levelsPavlovBiologyDNA, Protein Synthesis & Gene Technologyonly 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.

• 37Read a nuclide symbol into protons, neutrons and electronsPavlovChemistryAtomic Structure, Bonding & the Periodic Table5 other sittings

  Same method, other sittings

  20162017201920212023

  Worked solutionOpen the method
• 38Screen an equilibrium with the three Le Chatelier leversKey methodChemistryRates of Reaction & Energetics2 other sittings

  Same method, other sittings

  20162017

  Worked solutionOpen the method
• 39Aqueous electrolysis discharge rules: below hydrogen deposits, halide gives the halogenCathode is reduction, anode is oxidation, and electrons flow from plus to minus outsideKey methodChemistryElectrolysis & Metals5 other sittings

  Same method, other sittings

  20162017201920202022

  Worked solutionOpen the method

  AlsoCathode is reduction, anode is oxidation, and electrons flow from plus to minus outside
• 40Relative atomic mass as an abundance-weighted mean, forwards or reversedBound the relative atomic mass between the isotope masses to eliminate optionsElectron configuration gives the Period, the Group and the elementTunnelChemistryAtomic Structure, Bonding & the Periodic Table6 other sittings

  Same method, other sittings

  201620172020202120222023

  Worked solutionOpen the method

  AlsoBound the relative atomic mass between the isotope masses to eliminate optionsElectron configuration gives the Period, the Group and the element
• 41Pick the separation technique from the property that differsKey methodChemistryOrganic Chemistry, Separation & Analysis2 other sittings

  Same method, other sittings

  20192020

  Worked solutionOpen the method
• 42Aqueous electrolysis discharge rules: below hydrogen deposits, halide gives the halogenTrack the bulk solution, not just the electrodes: concentration and pH changesKey methodChemistryElectrolysis & Metals5 other sittings

  Same method, other sittings

  20162017202020222023

  Worked solutionOpen the method

  AlsoTrack the bulk solution, not just the electrodes: concentration and pH changes
• 43Judge a reaction as redox from its type before assigning oxidation numbersKey methodChemistryChemical Reactions, Acids, Bases & Redox5 other sittings

  Same method, other sittings

  20162017201920222023

  Worked solutionOpen the method
• 44Isoelectronic species share an electron count but differ in protonsKey methodChemistryAtomic Structure, Bonding & the Periodic Table2 other sittings

  Same method, other sittings

  20172021

  Worked solutionOpen the method
• 45Read the balanced equation directly in gas volumes, and track the change in the number of gas moleculesPin exactly what is asked: the examiner plants the natural intermediate value as an optionKey methodChemistryQuantitative Chemistry5 other sittings

  Same method, other sittings

  20162017202020212022

  Worked solutionOpen the method

  AlsoPin exactly what is asked: the examiner plants the natural intermediate value as an option
• 46Classify the structure and bonding, then read off the propertyKey methodChemistryAtomic Structure, Bonding & the Periodic Table6 other sittings

  Same method, other sittings

  201620192020202120222023

  Worked solutionOpen the method
• 47Purity and hydration are just multiplying factors: scale in the right direction and the pre-scaled value is the trapKey methodChemistryQuantitative Chemistry2 other sittings

  Same method, other sittings

  20192023

  Worked solutionOpen the method
• 48Recall Group 1 and Group 17 trends whole, without deriving themPavlovChemistryAtomic Structure, Bonding & the Periodic Table3 other sittings

  Same method, other sittings

  201720202021

  Worked solutionOpen the method
• 49Rf equals spot distance over solvent-front distanceInterpreting a chromatogram: purity, identity, mobility and phasesPavlovChemistryOrganic Chemistry, Separation & Analysis6 other sittings

  Same method, other sittings

  201620172019202020212022

  Worked solutionOpen the method

  AlsoInterpreting a chromatogram: purity, identity, mobility and phases
• 50Gas volume to moles: convert cm3 to dm3 before dividing by 24PavlovChemistryRates of Reaction & Energetics1 other sitting

  Same method, other sittings

  2022

  Worked solutionOpen the method
• 51Balance a skeleton equation one element at a time, most constrained firstTunnelChemistryChemical Reactions, Acids, Bases & Redox3 other sittings

  Same method, other sittings

  201920202021

  Worked solutionOpen the method
• 52Back titration: acid that reacted equals acid supplied minus acid left overTunnelChemistryQuantitative Chemistry1 other sitting

  Same method, other sittings

  2022

  Worked solutionOpen the method
• 53Ion charges fix the formula, the reacting ratio and the number of ionsLimiting reagent by moles over coefficient, and the graph plateau marks the exact stoichiometric pointPavlovChemistryQuantitative Chemistry4 other sittings

  Same method, other sittings

  2016201720212023

  Worked solutionOpen the method

  AlsoLimiting reagent by moles over coefficient, and the graph plateau marks the exact stoichiometric point
• 54Base heat and yield on the equilibrium amount, not the inputKey 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.

• 1Fill the two-way table for overlapping categoriesTunnelProb & StatsProbability & Statistics2 other sittings

  Same method, other sittings

  20162017

  Worked solutionOpen the method
• 23D solids: equating volumes and surface areas algebraicallyTunnelPureGeometry5 other sittings

  Same method, other sittings

  20192020202120222023

  Worked solutionOpen the method
• 3Implicit line equations: isolate y to find the gradient safelyPavlovPureCoordinate Geometry3 other sittings

  Same method, other sittings

  201920202023

  Worked solutionOpen the method
• 4Circle theorems: angle chasingTunnelPureGeometry2 other sittings

  Same method, other sittings

  20212023

  Worked solutionOpen the method
• 5Setting up the proportionality constantTunnelPureRatio & Proportion5 other sittings

  Same method, other sittings

  20162017202020222023

  Worked solutionOpen the method
• 6Scaling laws for similar figures and solidsPavlovPureRatio & Proportion4 other sittings

  Same method, other sittings

  2017201920222023

  Worked solutionOpen the method
• 7Convert means into absolute totalsPavlovProb & StatsProbability & Statistics4 other sittings

  Same method, other sittings

  2016202020222023

  Worked solutionOpen the method
• 8Areas bounded by intersecting lines: use an axis as the baseKey methodPureCoordinate Geometry3 other sittings

  Same method, other sittings

  201920202022

  Worked solutionOpen the method
• 9Scaling laws for similar figures and solidsPavlovPureRatio & Proportion4 other sittings

  Same method, other sittings

  2017201920222023

  Worked solutionOpen the method
• 10Regular polygons: work with exterior angles, not interiorPavlovPureGeometry3 other sittings

  Same method, other sittings

  201620172022

  Worked solutionOpen the method
• 11Successive percentage changes as a product of decimal multipliersPavlovPureRatio & Proportion5 other sittings

  Same method, other sittings

  20162019202020212022

  Worked solutionOpen the method
• 12Pick a convenient starting value for pure ratio and percentage problemsKey methodPureRatio & Proportion1 other sitting

  Same method, other sittings

  2023

  Worked solutionOpen the method
• 13Bearings: use back-bearings and subtraction to find anglesPavlovPureGeometry3 other sittings

  Same method, other sittings

  201620172019

  Worked solutionOpen the method
• 14Avoid overlapping regions in Venn diagramsTunnelProb & StatsProbability & Statisticsonly this sitting

  Same method, other sittings

  This one has only struck here so far.

  Worked solutionOpen the method
• 15Circle theorems: angle chasingTunnelPureGeometry2 other sittings

  Same method, other sittings

  20212023

  Worked solutionOpen the method
• 18Calculate the complement for at least one or complex conditionsKey methodProb & StatsProbability & Statistics1 other sitting

  Same method, other sittings

  2020

  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.

• 35Sector and Segment MechanicsTunnelPureTrigonometry2 other sittings

  Same method, other sittings

  20172021

  Worked solutionOpen the method
• 39Discarding the Extraneous RootThree Consecutive Terms EquationKey methodPureSequences & Series3 other sittings

  Same method, other sittings

  201620202023

  Worked solutionOpen the method

  AlsoThree Consecutive Terms Equation
• 45Distance relations: square both sides to avoid surdsPavlovPureCoordinate Geometryonly this sitting

  Same method, other sittings

  This one has only struck here so far.

  Worked solutionOpen the method
• 51Fundamental Theorem for Net ChangeTunnelPureIntegrationonly 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.

• 73Symmetric Limits and Even FunctionsArea Between Curves via Single IntegralPavlovPureIntegration2 other sittings

  Same method, other sittings

  20202023

  Worked solutionOpen the method

  AlsoArea Between Curves via Single Integral
• 75Factor Out, Never DividePavlovPureTrigonometry1 other sitting

  Same method, other sittings

  2020

  Worked solutionOpen the method
• 77Roots via the DiscriminantSystem to Single QuadraticKey methodPureAlgebra, Functions & Graphs3 other sittings

  Same method, other sittings

  201620212022

  Worked solutionOpen the method

  AlsoSystem to Single Quadratic
• 79AP Conditions to Linear RelationsTunnelPureSequences & Series1 other sitting

  Same method, other sittings

  2023

  Worked solutionOpen the method
• 81Sum of Logarithms → Product of ArgumentsPavlovPureExponentials & Logarithms1 other sitting

  Same method, other sittings

  2021

  Worked solutionOpen the method
• 83Extremes on a circle: use geometry, not calculusKey methodPureCoordinate Geometry1 other sitting

  Same method, other sittings

  2023

  Worked solutionOpen the method
• 85Look Ahead in Binomial ExpansionsKey methodPureSequences & Series2 other sittings

  Same method, other sittings

  20172023

  Worked solutionOpen the method
• 87Symbolic Evaluation of Parametric IntegralsTunnelPureIntegrationonly this sitting

  Same method, other sittings

  This one has only struck here so far.

  Worked solutionOpen the method
• 89Geometric OptimisationTunnelPureDifferentiation2 other sittings

  Same method, other sittings

  20162021

  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.

• 19Alpha and beta decay bookkeeping: fix the alpha count from the mass number firstTunnelPhysicsRadioactivity5 other sittings

  Same method, other sittings

  20162017201920202023

  Worked solutionOpen the method
• 20Label the circuit with the two invariants, then carry the one given number acrossIn a two state question, lock the invariant from state one before touching state twoTunnelPhysicsElectricity & Magnetism7 other sittings

  Same method, other sittings

  2016201720192020202120222023

  Worked solutionOpen the method

  AlsoIn a two state question, lock the invariant from state one before touching state two
• 21Classify a wave on the spectrum by converting band edges with the wave equationTunnelPhysicsWaves, Sound & the Electromagnetic Spectrum3 other sittings

  Same method, other sittings

  201720202021

  Worked solutionOpen the method
• 22Read a motion graph by gradient and area, one phase at a timeDistance, displacement and average speed are three different readings of the same motionTunnelPhysicsKinematics & Forces5 other sittings

  Same method, other sittings

  20162017201920202023

  Worked solutionOpen the method

  AlsoDistance, displacement and average speed are three different readings of the same motion
• 23Power is energy per second: convert the time first, and use force times velocity at constant speedEfficiency chain: multiply going out, divide coming back, never add the lossesPavlovPhysicsEnergy, Work & Momentum2 other sittings

  Same method, other sittings

  20172019

  Worked solutionOpen the method

  AlsoEfficiency chain: multiply going out, divide coming back, never add the losses
• 24Heat transfer is driven by the temperature difference alone, never by the individual temperaturesConduction along a bar: rate is proportional to area times temperature difference, divided by lengthWhich expression questions: settle them by units and by one variable at a timeKey methodPhysicsThermal Physics4 other sittings

  Same method, other sittings

  2016201920212022

  Worked solutionOpen the method

  AlsoConduction along a bar: rate is proportional to area times temperature difference, divided by lengthWhich expression questions: settle them by units and by one variable at a time
• 25Efficiency chain: multiply going out, divide coming back, never add the lossesTunnelPhysicsEnergy, Work & Momentum1 other sitting

  Same method, other sittings

  2017

  Worked solutionOpen the method
• 26The bridge between momentum and kinetic energyKey methodPhysicsEnergy, Work & Momentum2 other sittings

  Same method, other sittings

  20162019

  Worked solutionOpen the method
• 27Count-rate questions: subtract the background before reading anythingKey methodPhysicsRadioactivity1 other sitting

  Same method, other sittings

  2020

  Worked solutionOpen the method
• 28Free body first: list every force with its sign, then set the resultant equal to maTunnelPhysicsKinematics & Forces3 other sittings

  Same method, other sittings

  201620202023

  Worked solutionOpen the method
• 29Total path of an oscillating particle is four times the amplitude per cycleKey methodPhysicsWaves, Sound & the Electromagnetic Spectrum3 other sittings

  Same method, other sittings

  201620192023

  Worked solutionOpen the method
• 30Turn what you are given into a mass before you divide by a volumePavlovPhysicsMatter2 other sittings

  Same method, other sittings

  20162023

  Worked solutionOpen the method
• 31Read a motion graph by gradient and area, one phase at a timeSteady motion is a free equation, and after a perturbation only the change is unbalancedTunnelPhysicsKinematics & Forces6 other sittings

  Same method, other sittings

  201620172019202020212023

  Worked solutionOpen the method

  AlsoSteady motion is a free equation, and after a perturbation only the change is unbalanced
• 32Across a boundary the frequency is fixed, so speed and wavelength change togetherPavlovPhysicsWaves, Sound & the Electromagnetic Spectrum2 other sittings

  Same method, other sittings

  20202023

  Worked solutionOpen the method
• 33Alpha and beta decay bookkeeping: fix the alpha count from the mass number firstNuclear reactions: balance nucleon number and proton number across the arrowTunnelPhysicsRadioactivity5 other sittings

  Same method, other sittings

  20162017201920202023

  Worked solutionOpen the method

  AlsoNuclear reactions: balance nucleon number and proton number across the arrow
• 34When the resistances are identical or never given, call each one R and let it cancelChoose the power formula by the shared quantity, then split power as R or as one over RKey methodPhysicsElectricity & Magnetism6 other sittings

  Same method, other sittings

  201620172019202120222023

  Worked solutionOpen the method

  AlsoChoose the power formula by the shared quantity, then split power as R or as one over R
• 35In a two state question, lock the invariant from state one before touching state twoFor a non ohmic component the graph is its Ohm law: the circuit picks the axisKey methodPhysicsElectricity & Magnetism6 other sittings

  Same method, other sittings

  201620192020202120222023

  Worked solutionOpen the method

  AlsoFor a non ohmic component the graph is its Ohm law: the circuit picks the axis