AQA A-Level Chemistry Paper 3 Exam
Guide: 2025–2026 Edition with Complete
Solutions
Key apparatus for preparation of a standard solution? - answers Volumetric flask,
pipette and filler, burette, conical flask, white tile.
How do you ensure an accurate standard solution? - answers Use a mass balance to
weigh solute accurately, dissolve fully in deionised water, and rinse all equipment into
the volumetric flask before making up to the line.
Why is a white tile used in titrations? - answers To help detect the colour change more
clearly at the end point.
What indicators are used in acid-base titrations and why? - answers Phenolphthalein
(pink in alkali, colourless in acid) or methyl orange (yellow in alkali, red in acid) -
depending on strong/weak acid or base used.
How do you ensure accurate titration results? - answers Repeat until concordant titres
(within 0.1 cm³), swirl the flask, add dropwise near end point, rinse burette tip.
How is heat energy calculated? - answers ∆Q = mc∆T
Why might your result be less exothermic than expected when measuring enthalpy
change? - answers Heat loss to surroundings, incomplete combustion, evaporation of
fuel or solution.
How can calorimetry be improved? - answers Use a lid, insulate the container, use a
bomb calorimeter for combustion reactions.
What are the main sources of error in calorimetry? - answers Heat loss, inaccurate
temperature readings, assuming solution has same heat capacity as water.
What visual method is used in the thiosulfate reaction? - answers Time how long it
takes for a cross to disappear under the flask as sulfur precipitates.
How does increasing concentration affect rate? - answers Increases frequency of
collisions, leading to a faster rate.
What is a control variable in the thiosulfate experiment? - answers Temperature, volume
of solutions, same person judging the disappearance of the cross.
, Why is the reaction considered to have a subjective end point in sodium thiosulfate and
HCl reaction? - answers The disappearance of the cross depends on human judgment,
which can vary.
What test is used for sulfate ions? - answers Add HCl and BaCl₂ - white precipitate of
BaSO₄ forms.
What test is used for halide ions? - answers Add dilute then concentrated HNO₃ and
AgNO₃ - Cl⁻ (white ppt), Br⁻ (cream), I⁻ (yellow).
What test is used for carbonate ions? - answers Add HCl - effervescence due to CO₂,
which turns limewater cloudy.
What test is used for ammonium ions? - answers Add NaOH and warm - test gas with
damp red litmus (turns blue due to NH₃).
Why is nitric acid used instead of HCl in halide tests? - answers HCl contains Cl⁻ ions
which would interfere with the test.
What standard conditions are used in electrochemical cells? - answers 1.00 mol/dm³
solution, 298 K, 100 kPa pressure, and inert platinum electrode if needed.
What is the salt bridge for and what is it made of? - answers Allows ion flow to maintain
electrical neutrality; made of filter paper soaked in KNO₃.
Which direction do electrons flow in an electrochemical cell? - answers From the more
reactive metal (more negative E°) to the less reactive metal (more positive E°).
What is a standard hydrogen electrode? - answers Reference electrode with E° = 0 V,
used for measuring standard electrode potentials.
What makes a good recrystallisation solvent? - answers Dissolves the compound when
hot, not when cold; impurities either soluble when cold or insoluble always.
What steps are involved in recrystallisation? - answers Dissolve in hot solvent, filter if
needed, cool slowly to crystallise, filter off crystals, dry them.
How can purity of the solid be assessed in recrystallisation? - answers Measure melting
point and compare with literature value; pure compounds have sharp melting ranges.
Why might yield be low in recrystallisation? - answers Product remains dissolved in
solvent or lost during transfer.
What safety precautions are important in distillation? - answers Use anti-bumping
granules, avoid sealing apparatus, heat gently, collect correct fraction.
Guide: 2025–2026 Edition with Complete
Solutions
Key apparatus for preparation of a standard solution? - answers Volumetric flask,
pipette and filler, burette, conical flask, white tile.
How do you ensure an accurate standard solution? - answers Use a mass balance to
weigh solute accurately, dissolve fully in deionised water, and rinse all equipment into
the volumetric flask before making up to the line.
Why is a white tile used in titrations? - answers To help detect the colour change more
clearly at the end point.
What indicators are used in acid-base titrations and why? - answers Phenolphthalein
(pink in alkali, colourless in acid) or methyl orange (yellow in alkali, red in acid) -
depending on strong/weak acid or base used.
How do you ensure accurate titration results? - answers Repeat until concordant titres
(within 0.1 cm³), swirl the flask, add dropwise near end point, rinse burette tip.
How is heat energy calculated? - answers ∆Q = mc∆T
Why might your result be less exothermic than expected when measuring enthalpy
change? - answers Heat loss to surroundings, incomplete combustion, evaporation of
fuel or solution.
How can calorimetry be improved? - answers Use a lid, insulate the container, use a
bomb calorimeter for combustion reactions.
What are the main sources of error in calorimetry? - answers Heat loss, inaccurate
temperature readings, assuming solution has same heat capacity as water.
What visual method is used in the thiosulfate reaction? - answers Time how long it
takes for a cross to disappear under the flask as sulfur precipitates.
How does increasing concentration affect rate? - answers Increases frequency of
collisions, leading to a faster rate.
What is a control variable in the thiosulfate experiment? - answers Temperature, volume
of solutions, same person judging the disappearance of the cross.
, Why is the reaction considered to have a subjective end point in sodium thiosulfate and
HCl reaction? - answers The disappearance of the cross depends on human judgment,
which can vary.
What test is used for sulfate ions? - answers Add HCl and BaCl₂ - white precipitate of
BaSO₄ forms.
What test is used for halide ions? - answers Add dilute then concentrated HNO₃ and
AgNO₃ - Cl⁻ (white ppt), Br⁻ (cream), I⁻ (yellow).
What test is used for carbonate ions? - answers Add HCl - effervescence due to CO₂,
which turns limewater cloudy.
What test is used for ammonium ions? - answers Add NaOH and warm - test gas with
damp red litmus (turns blue due to NH₃).
Why is nitric acid used instead of HCl in halide tests? - answers HCl contains Cl⁻ ions
which would interfere with the test.
What standard conditions are used in electrochemical cells? - answers 1.00 mol/dm³
solution, 298 K, 100 kPa pressure, and inert platinum electrode if needed.
What is the salt bridge for and what is it made of? - answers Allows ion flow to maintain
electrical neutrality; made of filter paper soaked in KNO₃.
Which direction do electrons flow in an electrochemical cell? - answers From the more
reactive metal (more negative E°) to the less reactive metal (more positive E°).
What is a standard hydrogen electrode? - answers Reference electrode with E° = 0 V,
used for measuring standard electrode potentials.
What makes a good recrystallisation solvent? - answers Dissolves the compound when
hot, not when cold; impurities either soluble when cold or insoluble always.
What steps are involved in recrystallisation? - answers Dissolve in hot solvent, filter if
needed, cool slowly to crystallise, filter off crystals, dry them.
How can purity of the solid be assessed in recrystallisation? - answers Measure melting
point and compare with literature value; pure compounds have sharp melting ranges.
Why might yield be low in recrystallisation? - answers Product remains dissolved in
solvent or lost during transfer.
What safety precautions are important in distillation? - answers Use anti-bumping
granules, avoid sealing apparatus, heat gently, collect correct fraction.
