IGCSE Chemistry: Acids, Bases and Salts — Practice Questions

These are original questions written for Marlbridge, in the style and at the standard of the examination. They are not reproduced past-paper questions — examination boards hold copyright in their own papers. Use these alongside the official past papers available free from your board.

Related: Acids, Bases and Salts revision notes

Tier note: questions and parts marked (0620 Extended, 5070 required) go beyond 0620 Core. All other questions are answerable by a 0620 Core candidate.

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Questions

1. (0620 Extended, 5070 required) Define an acid and a base in terms of protons. [2]

2. State the colour of litmus, methyl orange and thymolphthalein in acid and in alkali. [3]

3. Complete and balance:

(a) HCl + NaOH → [1] (b) H₂SO₄ + CuO → [1] (c) HNO₃ + CaCO₃ → [2]

4. Describe how you would prepare pure, dry crystals of copper(II) sulfate from copper(II) oxide and dilute sulfuric acid, explaining why the solid oxide is added in excess rather than measured exactly. [5]

5. (0620 Extended, 5070 required) Describe how you would prepare a pure sample of lead(II) sulfate, an insoluble salt, by precipitation. [4]

6. 25.0 cm³ of sodium hydroxide required 20.0 cm³ of 0.100 mol dm⁻³ hydrochloric acid for neutralisation.

(a) Calculate the moles of HCl. [2] (b) Calculate the concentration of the sodium hydroxide. [3]

7. (0620 Extended, 5070 required) Explain the difference between a strong acid and a weak acid, giving an example of each. [3]

8. Classify each of the following oxides as acidic, basic or amphoteric: sulfur dioxide, calcium oxide, aluminium oxide, carbon dioxide. [4]

9. For each of the following salts, state whether it would be prepared by (i) reacting an acid with an excess insoluble solid, (ii) titration, or (iii) precipitation, giving a reason based on solubility: (a) potassium sulfate, (b) lead(II) sulfate, (c) copper(II) chloride made from copper(II) carbonate. [6]

10. Anhydrous copper(II) sulfate is white, while hydrated copper(II) sulfate is blue. Explain how this difference is used as a chemical test, and give the formula of the hydrated salt. (Naming the water as “water of crystallisation” is 0620 Extended, 5070 required.) [3]

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Answers

1. An acid is a proton (H⁺) donor [1]; a base is a proton acceptor [1].

2. Litmus: red in acid, blue in alkali [1]. Methyl orange: red in acid, yellow in alkali [1]. Thymolphthalein: colourless in acid, blue in alkali [1].

3. (a) HCl + NaOH → NaCl + H₂O [1]. (b) H₂SO₄ + CuO → CuSO₄ + H₂O [1]. (c) 2HNO₃ + CaCO₃ → Ca(NO₃)₂ + H₂O + CO₂ [1] correct products, [1] balanced.

4. Warm the dilute sulfuric acid [1]. Add copper(II) oxide until it is in excess and no more dissolves, showing all the acid has reacted [1]. Filter to remove the excess solid [1]. Evaporate the filtrate to the point of crystallisation, then leave to crystallise slowly [1]. Dry the crystals between filter paper [1].

5. Mix solutions of a soluble lead salt (lead(II) nitrate) and a soluble sulfate (sodium sulfate) [1]. Filter to collect the precipitate [1]. Wash it with distilled water to remove soluble impurities [1]. Dry it in a warm oven or between filter paper [1].

6. (a) n = cV = 0.100 × (20.0 ÷ 1000) [1] = 2.00 × 10⁻³ mol [1].

(b) 1:1 ratio, so n(NaOH) = 2.00 × 10⁻³ mol [1] c = n ÷ V = 2.00 × 10⁻³ ÷ 0.0250 [1] = 0.0800 mol dm⁻³ [1].

7. A strong acid fully dissociates into ions in water, e.g. hydrochloric acid [1] [1]; a weak acid only partially dissociates, e.g. ethanoic acid [1].

8. Sulfur dioxide — acidic (a non-metal oxide) [1]. Calcium oxide — basic (a metal oxide) [1]. Aluminium oxide — amphoteric, reacting with both acids and bases [1]. Carbon dioxide — acidic (a non-metal oxide) [1].

9. (a) Potassium sulfate — by titration [1], since both potassium hydroxide and sulfuric acid are soluble, so excess solid cannot be filtered off; the alkali must be measured exactly against the acid [1]. (b) Lead(II) sulfate — by precipitation [1], since lead(II) sulfate itself is insoluble (one of the exceptions to the sulfate solubility rule), so it is made by mixing two soluble solutions whose ions combine to form it [1]. (c) Copper(II) chloride — by reacting the acid with an excess insoluble solid [1], since copper(II) carbonate is insoluble and copper(II) chloride is soluble, so the excess unreacted carbonate can simply be filtered off after the reaction [1].

10. Adding water to white anhydrous copper(II) sulfate turns it blue, so it is used as a simple chemical test for the presence of water [1]; formally, the water becomes chemically combined within the crystal structure as water of crystallisation [1]. The hydrated formula is CuSO₄·5H₂O [1].

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Where marks are usually lost

• Not adding the solid in excess, or not filtering, in a soluble salt preparation.
• Forgetting to wash the precipitate in a precipitation preparation.
• Not converting cm³ to dm³ in titration calculations.
• Confusing strong with concentrated.
• Choosing titration for a salt where one reactant is actually insoluble, or vice versa — the choice of method always follows directly from the solubility of the salt and its reactants, not from habit.
• Forgetting that amphoteric oxides (aluminium and zinc) react with both acids and bases, and are worth memorising as a pair since they don’t follow the metal/non-metal pattern of the other oxides.
• Describing the anhydrous-to-hydrated colour change without naming water of crystallisation, or without giving the correct hydrated formula.

For condensed recall notes on this topic, see the Acids, Bases and Salts revision notes; for the full explanation with worked examples, see the Acids, Bases and Salts study guide.