Chemistry

1. 1.

   25.0 cm³ of 0.100 mol/dm³ sulfuric acid neutralises sodium hydroxide. Calculate the moles of sulfuric acid and the moles of sodium hydroxide required. Use H₂SO₄ + 2NaOH → Na₂SO₄ + 2H₂O.

   [4 marks]

   Answer explanation

   Draft walkthroughs are based on marking guidance, not independently verified derivations.

   1. List the given quantities and the requested unknown. Choose the relation that connects them, state any required assumptions, then substitute before rounding. Preserve exact expressions when the task asks for an exact result.
   2. Develop this part of the answer: Converts 25.0 cm³ to 0.0250 dm³. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
   3. Work through this mathematical step: Calculates n(H₂SO₄) = 0.100 × 0.0250 = 0.00250 mol. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
   4. Work through this mathematical step: Uses the 1:2 equation ratio. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
   5. Work through this mathematical step: Obtains n(NaOH) = 0.00500 mol. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
   6. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Convert cm³ to dm³ before using concentration in mol/dm³.

   Marking points

   • Converts 25.0 cm³ to 0.0250 dm³.
   • Calculates n(H₂SO₄) = 0.100 × 0.0250 = 0.00250 mol.
   • Uses the 1:2 equation ratio.
   • Obtains n(NaOH) = 0.00500 mol.

   Examiner tip: Convert cm³ to dm³ before using concentration in mol/dm³.

2. 2.

   Describe how to prepare pure, dry copper(II) sulfate crystals from dilute sulfuric acid and excess copper(II) oxide.

   [5 marks] · no calculator

   Answer explanation

   Draft walkthroughs are based on marking guidance, not independently verified derivations.

   1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
   2. Develop this part of the answer: Warm the dilute sulfuric acid. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
   3. Develop this part of the answer: Add copper(II) oxide until some remains unreacted. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
   4. Develop this part of the answer: Filter to remove excess copper(II) oxide. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
   5. Develop this part of the answer: Heat the filtrate to concentrate it, then allow it to cool and crystallise. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
   6. Develop this part of the answer: Filter and dry the crystals. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
   7. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: The insoluble base is added in excess so all acid is neutralised.

   Marking points

   • Warm the dilute sulfuric acid.
   • Add copper(II) oxide until some remains unreacted.
   • Filter to remove excess copper(II) oxide.
   • Heat the filtrate to concentrate it, then allow it to cool and crystallise.
   • Filter and dry the crystals.

   Examiner tip: The insoluble base is added in excess so all acid is neutralised.

3. 3.

   State the colour changes of litmus, methyl orange and phenolphthalein indicators when moving from acidic to alkaline solutions.

   [3 marks] · no calculator

   Answer explanation

   Draft walkthroughs are based on marking guidance, not independently verified derivations.

   1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
   2. Develop this part of the answer: States that litmus is red in acid and blue in alkali. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
   3. Develop this part of the answer: States that methyl orange is red in acid and yellow in alkali. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
   4. Develop this part of the answer: States that phenolphthalein is colourless in acid and pink/magenta in alkali. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
   5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Learn each indicator's colour in both acid and alkali separately — mixing up which colour belongs to which condition is a common error.

   Marking points

   • States that litmus is red in acid and blue in alkali.
   • States that methyl orange is red in acid and yellow in alkali.
   • States that phenolphthalein is colourless in acid and pink/magenta in alkali.

   Examiner tip: Learn each indicator's colour in both acid and alkali separately — mixing up which colour belongs to which condition is a common error.

4. 4.

   Define a strong acid and a weak acid in terms of dissociation, and state whether hydrochloric acid and ethanoic acid are strong or weak.

   [4 marks] · no calculator

   Answer explanation

   Draft walkthroughs are based on marking guidance, not independently verified derivations.

   1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
   2. Develop this part of the answer: Defines a strong acid as one that fully (completely) dissociates into ions in aqueous solution. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
   3. Develop this part of the answer: Defines a weak acid as one that only partially dissociates into ions in aqueous solution. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
   4. Develop this part of the answer: States that hydrochloric acid is a strong acid. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
   5. Develop this part of the answer: States that ethanoic acid is a weak acid. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
   6. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Strong/weak (degree of dissociation) is a different concept from concentrated/dilute (amount of acid dissolved) — do not confuse the two.

   Marking points

   • Defines a strong acid as one that fully (completely) dissociates into ions in aqueous solution.
   • Defines a weak acid as one that only partially dissociates into ions in aqueous solution.
   • States that hydrochloric acid is a strong acid.
   • States that ethanoic acid is a weak acid.

   Examiner tip: Strong/weak (degree of dissociation) is a different concept from concentrated/dilute (amount of acid dissolved) — do not confuse the two.

5. 5.

   Describe how you would prepare a pure, dry sample of soluble sodium chloride crystals by titration, starting from dilute hydrochloric acid and dilute sodium hydroxide.

   [5 marks] · no calculator

   Answer explanation

   Draft walkthroughs are based on marking guidance, not independently verified derivations.

   1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
   2. Develop this part of the answer: Titrate a known volume of sodium hydroxide against the acid, using an indicator, to find the exact volume of acid needed for neutralisation. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
   3. Develop this part of the answer: Repeat the titration without indicator, mixing the exact volumes of acid and alkali found. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
   4. Develop this part of the answer: Heat the resulting neutral solution to evaporate some water and concentrate it. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
   5. Develop this part of the answer: Allow the concentrated solution to cool so crystals form. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
   6. Develop this part of the answer: Filter the crystals and dry them, e.g. between sheets of filter paper. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
   7. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: An indicator must be avoided in the final combining step, since it would contaminate the pure salt crystals — that is why the titration is repeated exactly without it.

   Marking points

   • Titrate a known volume of sodium hydroxide against the acid, using an indicator, to find the exact volume of acid needed for neutralisation.
   • Repeat the titration without indicator, mixing the exact volumes of acid and alkali found.
   • Heat the resulting neutral solution to evaporate some water and concentrate it.
   • Allow the concentrated solution to cool so crystals form.
   • Filter the crystals and dry them, e.g. between sheets of filter paper.

   Examiner tip: An indicator must be avoided in the final combining step, since it would contaminate the pure salt crystals — that is why the titration is repeated exactly without it.

6. 6.

   Write the ionic equation for the neutralisation reaction between any strong acid and any strong alkali in aqueous solution.

   [2 marks] · no calculator

   Answer explanation

   Draft walkthroughs are based on marking guidance, not independently verified derivations.

   1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
   2. Develop this part of the answer: States the reactants as H⁺(aq) and OH⁻(aq). Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
   3. Work through this mathematical step: Writes the balanced ionic equation H⁺(aq) + OH⁻(aq) → H₂O(l). Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
   4. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: This same ionic equation applies to every strong acid-strong alkali neutralisation, regardless of which specific acid and alkali are used — the spectator ions simply differ.

   Marking points

   • States the reactants as H⁺(aq) and OH⁻(aq).
   • Writes the balanced ionic equation H⁺(aq) + OH⁻(aq) → H₂O(l).

   Examiner tip: This same ionic equation applies to every strong acid-strong alkali neutralisation, regardless of which specific acid and alkali are used — the spectator ions simply differ.

7. 7.

   Explain why a solution with pH 3 is ten times more acidic than a solution with pH 4, in terms of hydrogen ion concentration.

   [3 marks] · no calculator

   Answer explanation

   Draft walkthroughs are based on marking guidance, not independently verified derivations.

   1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
   2. Work through this mathematical step: States that the pH scale is logarithmic, based on hydrogen ion concentration. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
   3. Develop this part of the answer: States that each single decrease of one pH unit corresponds to a ten-fold increase in hydrogen ion concentration. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
   4. Develop this part of the answer: States that since pH 3 is one unit lower than pH 4, the hydrogen ion concentration at pH 3 is ten times greater. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
   5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: A difference of 2 pH units corresponds to a 100-fold (10²) difference in hydrogen ion concentration, not a 20-fold difference — remember the scale is logarithmic, not linear.

   Marking points

   • States that the pH scale is logarithmic, based on hydrogen ion concentration.
   • States that each single decrease of one pH unit corresponds to a ten-fold increase in hydrogen ion concentration.
   • States that since pH 3 is one unit lower than pH 4, the hydrogen ion concentration at pH 3 is ten times greater.

   Examiner tip: A difference of 2 pH units corresponds to a 100-fold (10²) difference in hydrogen ion concentration, not a 20-fold difference — remember the scale is logarithmic, not linear.

8. 8.

   Describe a test to confirm the presence of carbonate ions in a solid sample, and state the expected observation.

   [3 marks] · no calculator

   Answer explanation

   Draft walkthroughs are based on marking guidance, not independently verified derivations.

   1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
   2. Develop this part of the answer: Adds a dilute acid (e.g. dilute hydrochloric acid) to the sample. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
   3. Develop this part of the answer: States the observation of effervescence (bubbling) as carbon dioxide gas is produced. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
   4. Develop this part of the answer: Confirms the gas is carbon dioxide by bubbling it through limewater, which turns milky/cloudy. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
   5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: The limewater test (turning milky/cloudy) is the standard confirmatory test for carbon dioxide gas specifically — do not confuse it with tests for other gases.

   Marking points

   • Adds a dilute acid (e.g. dilute hydrochloric acid) to the sample.
   • States the observation of effervescence (bubbling) as carbon dioxide gas is produced.
   • Confirms the gas is carbon dioxide by bubbling it through limewater, which turns milky/cloudy.

   Examiner tip: The limewater test (turning milky/cloudy) is the standard confirmatory test for carbon dioxide gas specifically — do not confuse it with tests for other gases.

9. 9.

   Distinguish between a soluble salt and an insoluble salt, and outline the different method used to prepare each from its constituent ions.

   [4 marks] · no calculator

   Answer explanation

   Draft walkthroughs are based on marking guidance, not independently verified derivations.

   1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
   2. Develop this part of the answer: States that a soluble salt dissolves in water, while an insoluble salt does not. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
   3. Develop this part of the answer: Outlines that soluble salts are typically prepared by reacting an acid with an appropriate base (or metal or carbonate), then evaporating and crystallising the resulting solution. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
   4. Develop this part of the answer: Outlines that insoluble salts are typically prepared by precipitation: mixing two soluble solutions whose ions combine to form the insoluble salt directly. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
   5. Develop this part of the answer: States that the insoluble salt precipitate is separated from the reaction mixture by filtration, then washed and dried. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
   6. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: The solubility of the target salt is what determines the preparation method: soluble salts require evaporation/crystallisation, insoluble salts require precipitation and filtration.

   Marking points

   • States that a soluble salt dissolves in water, while an insoluble salt does not.
   • Outlines that soluble salts are typically prepared by reacting an acid with an appropriate base (or metal or carbonate), then evaporating and crystallising the resulting solution.
   • Outlines that insoluble salts are typically prepared by precipitation: mixing two soluble solutions whose ions combine to form the insoluble salt directly.
   • States that the insoluble salt precipitate is separated from the reaction mixture by filtration, then washed and dried.

   Examiner tip: The solubility of the target salt is what determines the preparation method: soluble salts require evaporation/crystallisation, insoluble salts require precipitation and filtration.

10. 10.

    State the general word equation for the reaction between a metal and an acid, and identify the gas produced.

    [3 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
    2. Work through this mathematical step: States the word equation: metal + acid → salt + hydrogen. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
    3. Develop this part of the answer: Identifies the gas produced as hydrogen. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    4. Develop this part of the answer: States that hydrogen gas is confirmed with a lit splint, which produces a squeaky pop. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: The 'squeaky pop' test with a lit splint is the standard confirmatory test for hydrogen gas specifically.

    Marking points

    • States the word equation: metal + acid → salt + hydrogen.
    • Identifies the gas produced as hydrogen.
    • States that hydrogen gas is confirmed with a lit splint, which produces a squeaky pop.

    Examiner tip: The 'squeaky pop' test with a lit splint is the standard confirmatory test for hydrogen gas specifically.

11. 11.

    Classify each of the following oxides as acidic, basic or amphoteric: sulfur dioxide (SO₂), calcium oxide (CaO), and aluminium oxide (Al₂O₃). State one property that identifies an amphoteric oxide.

    [4 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
    2. Develop this part of the answer: States that sulfur dioxide, SO₂, a non-metal oxide, is acidic. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    3. Develop this part of the answer: States that calcium oxide, CaO, a metal oxide, is basic. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    4. Develop this part of the answer: States that aluminium oxide, Al₂O₃, is amphoteric. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    5. Develop this part of the answer: States that an amphoteric oxide reacts with both acids and bases (alkalis). Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    6. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: As a general rule, metal oxides are basic and non-metal oxides are acidic, but a few metal oxides (like aluminium oxide and zinc oxide) are exceptions and behave as amphoteric.

    Marking points

    • States that sulfur dioxide, SO₂, a non-metal oxide, is acidic.
    • States that calcium oxide, CaO, a metal oxide, is basic.
    • States that aluminium oxide, Al₂O₃, is amphoteric.
    • States that an amphoteric oxide reacts with both acids and bases (alkalis).

    Examiner tip: As a general rule, metal oxides are basic and non-metal oxides are acidic, but a few metal oxides (like aluminium oxide and zinc oxide) are exceptions and behave as amphoteric.

12. 12.

    Describe a test to confirm the presence of ammonia gas, and state the observation.

    [2 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
    2. Develop this part of the answer: States that damp red litmus paper is held near the gas. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    3. Develop this part of the answer: States the observation: the damp red litmus paper turns blue, confirming an alkaline gas (ammonia). Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    4. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Ammonia is the only common laboratory gas that turns damp red litmus paper blue — this makes the test unambiguous.

    Marking points

    • States that damp red litmus paper is held near the gas.
    • States the observation: the damp red litmus paper turns blue, confirming an alkaline gas (ammonia).

    Examiner tip: Ammonia is the only common laboratory gas that turns damp red litmus paper blue — this makes the test unambiguous.

13. 13.

    Black copper(II) oxide is added in excess to warm dilute sulfuric acid: CuO + H₂SO₄ → CuSO₄ + H₂O. State what would be observed as the oxide is added, and explain how you would know when the reaction is complete.

    [3 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
    2. Develop this part of the answer: States that the black solid gradually dissolves (disappears) as it reacts with the acid. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    3. Develop this part of the answer: States that the solution turns blue, since copper(II) sulfate forms a blue solution. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    4. Develop this part of the answer: States that the reaction is complete once black solid remains undissolved (in excess), showing that no acid remains to react further. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Adding an insoluble base in excess and then filtering off the leftover solid is a reliable way to ensure all the acid has reacted, without needing an indicator.

    Marking points

    • States that the black solid gradually dissolves (disappears) as it reacts with the acid.
    • States that the solution turns blue, since copper(II) sulfate forms a blue solution.
    • States that the reaction is complete once black solid remains undissolved (in excess), showing that no acid remains to react further.

    Examiner tip: Adding an insoluble base in excess and then filtering off the leftover solid is a reliable way to ensure all the acid has reacted, without needing an indicator.

14. 14.

    Describe a test to confirm the presence of sulfate ions in solution, including the reagents used and the expected observation.

    [3 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
    2. Develop this part of the answer: States that dilute hydrochloric acid is added first, followed by barium chloride (or barium nitrate) solution. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    3. Develop this part of the answer: States that the dilute acid is added first to remove any interference from carbonate ions, which would otherwise also give a precipitate with barium ions. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    4. Develop this part of the answer: States the observation: a white precipitate (barium sulfate) forms. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Adding dilute acid before the barium reagent is essential — without it, carbonate ions in the sample would also produce a white precipitate, giving a false-positive result.

    Marking points

    • States that dilute hydrochloric acid is added first, followed by barium chloride (or barium nitrate) solution.
    • States that the dilute acid is added first to remove any interference from carbonate ions, which would otherwise also give a precipitate with barium ions.
    • States the observation: a white precipitate (barium sulfate) forms.

    Examiner tip: Adding dilute acid before the barium reagent is essential — without it, carbonate ions in the sample would also produce a white precipitate, giving a false-positive result.

15. 15.

    Describe a test to confirm the presence of chloride ions in solution, including the reagents used and the expected observation.

    [3 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
    2. Develop this part of the answer: States that dilute nitric acid is added first, followed by silver nitrate solution. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    3. Develop this part of the answer: States that the dilute nitric acid is added first to remove interference from carbonate ions. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    4. Develop this part of the answer: States the observation: a white precipitate (silver chloride) forms. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Nitric acid, not hydrochloric or sulfuric acid, must be used to acidify the sample before this test, since adding chloride or sulfate ions from another acid would contaminate the result.

    Marking points

    • States that dilute nitric acid is added first, followed by silver nitrate solution.
    • States that the dilute nitric acid is added first to remove interference from carbonate ions.
    • States the observation: a white precipitate (silver chloride) forms.

    Examiner tip: Nitric acid, not hydrochloric or sulfuric acid, must be used to acidify the sample before this test, since adding chloride or sulfate ions from another acid would contaminate the result.

16. 16.

    Describe how a pure sample of insoluble lead(II) sulfate could be prepared from solutions of lead(II) nitrate and sodium sulfate, and write the ionic equation, including state symbols, for the reaction.

    [5 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
    2. Develop this part of the answer: States that solutions of lead(II) nitrate and sodium sulfate are mixed together. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    3. Develop this part of the answer: States that a precipitate of lead(II) sulfate forms immediately. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    4. Develop this part of the answer: States that the mixture is filtered to separate the lead(II) sulfate precipitate. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    5. Develop this part of the answer: States that the precipitate is washed with distilled water, then dried. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    6. Work through this mathematical step: Writes the ionic equation: Pb²⁺(aq) + SO₄²⁻(aq) → PbSO₄(s). Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
    7. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Precipitation reactions can be written as a simple ionic equation because only the two ions that combine to form the insoluble solid actually take part — the spectator ions (Na⁺ and NO₃⁻ here) are left out.

    Marking points

    • States that solutions of lead(II) nitrate and sodium sulfate are mixed together.
    • States that a precipitate of lead(II) sulfate forms immediately.
    • States that the mixture is filtered to separate the lead(II) sulfate precipitate.
    • States that the precipitate is washed with distilled water, then dried.
    • Writes the ionic equation: Pb²⁺(aq) + SO₄²⁻(aq) → PbSO₄(s).

    Examiner tip: Precipitation reactions can be written as a simple ionic equation because only the two ions that combine to form the insoluble solid actually take part — the spectator ions (Na⁺ and NO₃⁻ here) are left out.

17. 17.

    State one advantage of using a universal indicator or a pH meter over a single indicator such as litmus, when investigating the acidity or alkalinity of a solution.

    [2 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
    2. Develop this part of the answer: States that a universal indicator or pH meter can give an estimate of the actual pH value across the whole scale (0-14), not just whether a solution is acidic or alkaline. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    3. Develop this part of the answer: States that litmus (or a single indicator) only shows whether a solution is acidic or alkaline, without indicating how strongly acidic or alkaline it is. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    4. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: A pH meter gives the most precise reading of all, since it reports a numerical pH value directly rather than relying on matching a colour by eye.

    Marking points

    • States that a universal indicator or pH meter can give an estimate of the actual pH value across the whole scale (0-14), not just whether a solution is acidic or alkaline.
    • States that litmus (or a single indicator) only shows whether a solution is acidic or alkaline, without indicating how strongly acidic or alkaline it is.

    Examiner tip: A pH meter gives the most precise reading of all, since it reports a numerical pH value directly rather than relying on matching a colour by eye.

18. 18.

    State what is meant by the basicity of an acid, and state the basicity of hydrochloric acid and of sulfuric acid.

    [3 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
    2. Develop this part of the answer: States that basicity is the number of hydrogen ions (H⁺) that one molecule of the acid can release (donate) in solution. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    3. Develop this part of the answer: States that hydrochloric acid, HCl, has a basicity of 1 (it is monoprotic/monobasic). Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    4. Develop this part of the answer: States that sulfuric acid, H₂SO₄, has a basicity of 2 (it is diprotic/dibasic). Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Basicity determines the mole ratio in a neutralisation equation: a diprotic acid like sulfuric acid needs twice as many moles of a monobasic alkali (like NaOH) as a monoprotic acid would.

    Marking points

    • States that basicity is the number of hydrogen ions (H⁺) that one molecule of the acid can release (donate) in solution.
    • States that hydrochloric acid, HCl, has a basicity of 1 (it is monoprotic/monobasic).
    • States that sulfuric acid, H₂SO₄, has a basicity of 2 (it is diprotic/dibasic).

    Examiner tip: Basicity determines the mole ratio in a neutralisation equation: a diprotic acid like sulfuric acid needs twice as many moles of a monobasic alkali (like NaOH) as a monoprotic acid would.

19. 19.

    Describe a test to confirm the presence of ammonium ions in a solid sample.

    [3 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
    2. Develop this part of the answer: States that aqueous sodium hydroxide is added to the sample and the mixture is warmed gently. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    3. Develop this part of the answer: States the observation: a gas with a pungent smell is produced. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    4. Develop this part of the answer: States that the gas is confirmed as ammonia by holding damp red litmus paper near the mouth of the test tube, which turns blue. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Warming is essential in this test — without heat, the reaction between the ammonium salt and sodium hydroxide to release ammonia gas proceeds too slowly to detect.

    Marking points

    • States that aqueous sodium hydroxide is added to the sample and the mixture is warmed gently.
    • States the observation: a gas with a pungent smell is produced.
    • States that the gas is confirmed as ammonia by holding damp red litmus paper near the mouth of the test tube, which turns blue.

    Examiner tip: Warming is essential in this test — without heat, the reaction between the ammonium salt and sodium hydroxide to release ammonia gas proceeds too slowly to detect.

20. 20.

    A student mixes dilute hydrochloric acid with dilute sodium hydroxide solution in an insulated cup and measures a rise in temperature. State the type of reaction this shows neutralisation to be, and state what would happen to the temperature rise if the experiment were repeated using half the volume of each solution (at the same concentrations).

    [3 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
    2. Develop this part of the answer: States that neutralisation is an exothermic reaction, since heat energy is released to the surroundings, causing the temperature to rise. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    3. Develop this part of the answer: States that the temperature rise would stay approximately the same. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    4. Develop this part of the answer: Explains that although halving the volumes releases half as much heat energy, this heat also warms a proportionally smaller mass of solution, so the temperature change is unaffected. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Temperature change depends on energy released per unit mass of solution, not on the total energy released — this is why proportionally scaling all volumes down leaves the temperature change unchanged.

    Marking points

    • States that neutralisation is an exothermic reaction, since heat energy is released to the surroundings, causing the temperature to rise.
    • States that the temperature rise would stay approximately the same.
    • Explains that although halving the volumes releases half as much heat energy, this heat also warms a proportionally smaller mass of solution, so the temperature change is unaffected.

    Examiner tip: Temperature change depends on energy released per unit mass of solution, not on the total energy released — this is why proportionally scaling all volumes down leaves the temperature change unchanged.

21. 21.

    Calcium carbonate reacts with dilute hydrochloric acid: CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂. State whether the calcium chloride salt formed is soluble or insoluble, and state two observations you would expect to see during this reaction.

    [3 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
    2. Develop this part of the answer: States that the calcium chloride salt formed is soluble. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    3. Develop this part of the answer: States the observation of effervescence (bubbling), as carbon dioxide gas is produced. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    4. Develop this part of the answer: States that the solid calcium carbonate gradually dissolves (disappears) as the reaction proceeds. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Almost all common chlorides are soluble (with a few notable exceptions such as silver chloride and lead(II) chloride) — this is a useful solubility rule to remember.

    Marking points

    • States that the calcium chloride salt formed is soluble.
    • States the observation of effervescence (bubbling), as carbon dioxide gas is produced.
    • States that the solid calcium carbonate gradually dissolves (disappears) as the reaction proceeds.

    Examiner tip: Almost all common chlorides are soluble (with a few notable exceptions such as silver chloride and lead(II) chloride) — this is a useful solubility rule to remember.

22. 22.

    State the difference between the terms 'anhydrous' and 'hydrated' as applied to salts, giving an example of each for copper(II) sulfate.

    [3 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
    2. Develop this part of the answer: States that 'anhydrous' means without any water of crystallisation. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    3. Develop this part of the answer: States that 'hydrated' means containing a fixed amount of water of crystallisation within its crystal structure. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    4. Develop this part of the answer: States the example: anhydrous copper(II) sulfate is white, while hydrated copper(II) sulfate, CuSO₄·5H₂O, is blue. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: The colour change of anhydrous (white) copper(II) sulfate turning blue is a standard test for the presence of water, since it reacts specifically and visibly with water to reform the hydrated salt.

    Marking points

    • States that 'anhydrous' means without any water of crystallisation.
    • States that 'hydrated' means containing a fixed amount of water of crystallisation within its crystal structure.
    • States the example: anhydrous copper(II) sulfate is white, while hydrated copper(II) sulfate, CuSO₄·5H₂O, is blue.

    Examiner tip: The colour change of anhydrous (white) copper(II) sulfate turning blue is a standard test for the presence of water, since it reacts specifically and visibly with water to reform the hydrated salt.

23. 23.

    Silver nitrate solution is mixed with sodium chloride solution, forming a precipitate. Write the full ionic equation, including state symbols, for this reaction, and name the precipitate formed.

    [3 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
    2. Work through this mathematical step: Identifies that sodium ions and nitrate ions are spectator ions and are not included in the ionic equation. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
    3. Work through this mathematical step: Writes the ionic equation: Ag⁺(aq) + Cl⁻(aq) → AgCl(s). Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
    4. Develop this part of the answer: Names the precipitate as silver chloride. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: A spectator ion appears unchanged on both sides of the full equation, so it can always be cancelled out to leave only the ions that actually react in the ionic equation.

    Marking points

    • Identifies that sodium ions and nitrate ions are spectator ions and are not included in the ionic equation.
    • Writes the ionic equation: Ag⁺(aq) + Cl⁻(aq) → AgCl(s).
    • Names the precipitate as silver chloride.

    Examiner tip: A spectator ion appears unchanged on both sides of the full equation, so it can always be cancelled out to leave only the ions that actually react in the ionic equation.

24. 24.

    Calculate the volume of 0.20 mol/dm³ hydrochloric acid required to react exactly with 20.0 cm³ of 0.15 mol/dm³ calcium hydroxide solution: Ca(OH)₂ + 2HCl → CaCl₂ + 2H₂O.

    [4 marks]

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. List the given quantities and the requested unknown. Choose the relation that connects them, state any required assumptions, then substitute before rounding. Preserve exact expressions when the task asks for an exact result.
    2. Work through this mathematical step: Calculates moles of Ca(OH)₂ = 0.15 × 0.0200 = 0.0030 mol. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
    3. Work through this mathematical step: Uses the 1:2 equation ratio to state moles of HCl = 0.0030 × 2 = 0.0060 mol. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
    4. Work through this mathematical step: Uses volume = moles ÷ concentration. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
    5. Work through this mathematical step: Obtains volume = 0.0060 ÷ 0.20 = 0.030 dm³ (30 cm³). Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
    6. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Calcium hydroxide releases two hydroxide ions per formula unit, so it needs twice as many moles of a monoprotic acid like HCl for complete neutralisation.

    Marking points

    • Calculates moles of Ca(OH)₂ = 0.15 × 0.0200 = 0.0030 mol.
    • Uses the 1:2 equation ratio to state moles of HCl = 0.0030 × 2 = 0.0060 mol.
    • Uses volume = moles ÷ concentration.
    • Obtains volume = 0.0060 ÷ 0.20 = 0.030 dm³ (30 cm³).

    Examiner tip: Calcium hydroxide releases two hydroxide ions per formula unit, so it needs twice as many moles of a monoprotic acid like HCl for complete neutralisation.

25. 25.

    A strong acid of pH 2 is diluted with water so that its concentration decreases by a factor of 10. State the new pH of the diluted acid, and explain your reasoning in terms of hydrogen ion concentration.

    [3 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
    2. Develop this part of the answer: States that the new pH is 3. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    3. Develop this part of the answer: Explains that diluting the concentration by a factor of 10 decreases the hydrogen ion concentration by the same factor of 10. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    4. Work through this mathematical step: States that since the pH scale is logarithmic, a ten-fold decrease in hydrogen ion concentration corresponds to an increase of exactly one pH unit. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
    5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: For a strong acid specifically, diluting by a factor of 10 always changes the pH by exactly 1 unit, since the acid is fully dissociated and hydrogen ion concentration scales directly with the acid's concentration.

    Marking points

    • States that the new pH is 3.
    • Explains that diluting the concentration by a factor of 10 decreases the hydrogen ion concentration by the same factor of 10.
    • States that since the pH scale is logarithmic, a ten-fold decrease in hydrogen ion concentration corresponds to an increase of exactly one pH unit.

    Examiner tip: For a strong acid specifically, diluting by a factor of 10 always changes the pH by exactly 1 unit, since the acid is fully dissociated and hydrogen ion concentration scales directly with the acid's concentration.

Original, unofficial practice questions from SubjectScout — not official past-paper questions.