Paper 4 Acids and Bases Structured Practice
5 structured exam questions on acids, bases, and salts for IGCSE Chemistry Paper 4. Covers neutralisation, salt preparation methods, oxide classification, titration, and strong vs weak acids. Total: 26 marks.
Published by IGCSEChemistry.com.my
Chemistry teaching team: K. S. Tan (15+ years teaching IGCSE Chemistry) and Ms Yash (10+ years teaching IGCSE Chemistry) and Ms Kartini (15+ years teaching IGCSE Chemistry).
Mapped to Cambridge IGCSE Chemistry 0620 (2026–2028). Last updated 2026-08-19.
Five structured questions in the style of IGCSE Chemistry Paper 4, totalling 26 marks. Write your answers in full before checking the mark scheme.
Question 1 (6 marks)
A student prepares a sample of copper(II) sulfate crystals by reacting copper(II) oxide with dilute sulfuric acid.
(a) Write the balanced equation for this reaction. [1]
(b) Describe the method the student should use to prepare pure copper(II) sulfate crystals. [4]
(c) State the colour of the crystals obtained. [1]
Mark scheme
(a) CuO + H₂SO₄ → CuSO₄ + H₂O [1]
(b) Add excess copper(II) oxide to warm dilute sulfuric acid and stir [1]. The black powder reacts and the solution turns blue [1]. Filter the mixture to remove the excess (unreacted) copper(II) oxide [1]. Heat the blue filtrate gently to evaporate some water, then leave to cool and allow crystals to form / crystallise [1].
(c) Blue [1]
Total: 6 marks
Question 2 (5 marks)
Classify each of the following oxides as acidic, basic, neutral, or amphoteric.
(a) Calcium oxide, CaO [1]
(b) Carbon dioxide, CO₂ [1]
(c) Aluminium oxide, Al₂O₃ [1]
(d) Write the equation for the reaction of carbon dioxide with sodium hydroxide solution. [1]
(e) Explain why aluminium oxide is described as amphoteric. [1]
Mark scheme
(a) Basic [1]
(b) Acidic [1]
(c) Amphoteric [1]
(d) CO₂ + 2NaOH → Na₂CO₃ + H₂O [1]
(e) It can react with both acids and bases to form a salt and water [1]. For example, it reacts with HCl to form AlCl₃ + H₂O, and with NaOH to form NaAlO₂ + H₂O.
Total: 5 marks
Question 3 (5 marks)
(a) Define the term strong acid. [1]
(b) Define the term weak acid. [1]
(c) A student has two solutions of the same concentration: 0.1 mol/dm³ hydrochloric acid and 0.1 mol/dm³ ethanoic acid. Compare the pH and the rate of reaction with magnesium for these two solutions. [3]
Mark scheme
(a) A strong acid is one that completely ionises / dissociates in water / 100% of the molecules form H⁺ ions [1].
(b) A weak acid is one that only partially ionises / dissociates in water / only a small proportion of molecules form H⁺ ions [1].
(c) The hydrochloric acid has a lower pH than the ethanoic acid (at the same concentration) [1]. This is because HCl is fully ionised, producing more H⁺ ions [1]. Magnesium reacts faster with the hydrochloric acid because there is a higher concentration of H⁺ ions available to collide with the magnesium [1].
Total: 5 marks
Question 4 (5 marks)
A student carries out a titration to prepare pure sodium chloride. 25.0 cm³ of sodium hydroxide solution is placed in a conical flask. Dilute hydrochloric acid is added from a burette until the indicator shows the end point.
(a) Write the equation for the reaction. [1]
(b) Explain why titration is used to prepare sodium chloride instead of the excess solid method. [1]
(c) After finding the exact volume of acid needed, the student repeats the titration without the indicator. Explain why. [1]
(d) Describe how the student obtains pure dry sodium chloride crystals from the solution. [2]
Mark scheme
(a) NaOH + HCl → NaCl + H₂O [1]
(b) Both NaOH and HCl are soluble, so there is no excess solid to filter off / the excess solid method only works when one reactant is insoluble [1].
(c) The indicator would contaminate the salt / the sodium chloride would be impure if indicator were present [1].
(d) Heat the solution gently to evaporate the water [1]. Continue heating until dry / until all water has evaporated to leave white crystalline solid / evaporate to dryness [1].
Total: 5 marks
Question 5 (5 marks)
Lead(II) chloride is an insoluble salt. It can be prepared by a precipitation reaction.
(a) State the solutions that should be mixed to produce lead(II) chloride by precipitation. [2]
(b) Write the ionic equation for this precipitation reaction. [1]
(c) Describe how to obtain pure, dry lead(II) chloride from the reaction mixture. [2]
Mark scheme
(a) Lead(II) nitrate solution [1] and sodium chloride solution (or any soluble chloride, e.g. potassium chloride, hydrochloric acid) [1].
(b) Pb²⁺(aq) + 2Cl⁻(aq) → PbCl₂(s) [1]
(c) Filter the mixture to collect the precipitate as the residue on the filter paper [1]. Wash the residue with distilled water and dry between sheets of filter paper / in a warm oven [1].
Total: 5 marks
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