Chemistry 0620 · syllabus 7, 12.2
Cambridge IGCSE Chemistry 0620 — Acids, bases and salts topical past papers
Reactions of acids and bases, indicators and pH, strong and weak acids, acidic, basic and amphoteric oxides, preparing salts, solubility rules and titration.
- Questions
- 879
- Years
- 2009–2026
- With mark scheme
- 99%
- Questions 2022–2026
- 310
Anyone can browse these pages. Opening the questions and their mark schemes takes a free Momentum account.
What Acids, bases and salts covers
Acids react with metals to give a salt and hydrogen, with bases to give a salt and water, and with carbonates to give a salt, water and carbon dioxide. Bases are the oxides and hydroxides of metals, alkalis are the bases that dissolve in water, and bases release ammonia when warmed with ammonium salts. Aqueous acids contain H⁺ ions and alkalis OH⁻ ions, and neutralisation is H⁺(aq) + OH⁻(aq) → H₂O(l). Indicators tell them apart: litmus is red in acid and blue in alkali, methyl orange red in acid and yellow in alkali, and thymolphthalein colourless in acid and blue in alkali, while universal indicator gives a colour matching a pH, below 7 acidic, 7 neutral and above 7 alkaline.
Most oxides of non-metals, such as SO₂ and CO₂, are acidic, and oxides of metals, such as CuO and CaO, are basic. The Extended route adds amphoteric oxides, Al₂O₃ and ZnO, which react with both acids and bases; acids as proton donors and bases as proton acceptors; and strong and weak acids. Hydrochloric acid is completely dissociated in water and ethanoic acid only partly, so at the same concentration ethanoic acid contains fewer H⁺ ions and has a higher pH.
A soluble salt is made from an acid and an alkali by titration, or from an acid and an excess of a metal, an insoluble base or an insoluble carbonate, the excess being filtered off before the solution is crystallised. On the Extended route an insoluble salt is made by mixing two solutions to form a precipitate, which is filtered, washed and dried. Choosing the method depends on the solubility rules: all sodium, potassium, ammonium and nitrate salts are soluble; chlorides are soluble except silver and lead chloride, and sulfates except barium, calcium and lead sulfate; carbonates and hydroxides are insoluble except those of sodium, potassium and ammonium, with calcium hydroxide partly soluble. Titration itself — a volumetric pipette, a burette and an indicator that changes colour at the end-point — belongs here, and both salt preparation and titration are standard practical contexts.
In the syllabus
The headings this topic has in the Cambridge International 2026–2028 syllabus, as the board prints them.
- 7 Acids, bases and salts
- 7.1 The characteristic properties of acids and bases
- 7.2 Oxides
- 7.3 Preparation of salts
- 12.2 Acid–base titrations
What you should be able to do
- Describe the reactions of acids with metals, bases and carbonates, and of bases with acids and ammonium salts, with equations
- Give the colours of litmus, methyl orange and thymolphthalein in acid and in alkali, and compare acidity using universal indicator and pH
- Classify oxides as acidic or basic, and as amphoteric on the Extended route
- Describe how to prepare a pure, dry soluble salt by each method, and on the Extended route an insoluble salt by precipitation
- Describe an acid–base titration, including the apparatus and how the end-point is recognised
- Explain strong and weak acids by their dissociation, and acids and bases as proton donors and acceptors (Extended)
Key ideas
- Acid + metal → salt + hydrogen; acid + base → salt + water; acid + carbonate → salt + water + carbon dioxide
- Neutralisation: H⁺(aq) + OH⁻(aq) → H₂O(l)
- Strong acid, fully dissociated: HCl(aq) → H⁺(aq) + Cl⁻(aq); weak acid, partly dissociated: CH₃COOH(aq) ⇌ H⁺(aq) + CH₃COO⁻(aq)
- Hydrated salts contain water of crystallisation, as in CuSO₄·5H₂O; anhydrous salts contain none
- Soluble salt from an insoluble base: add the base in excess to warm acid, filter, heat the filtrate until crystals start to form, cool, then filter and dry the crystals
How Acids, bases and salts has been examined
From 2022 to 2026, Acids, bases and salts was on Paper 4 · Theory (Extended) in 11 of 14 exam series, worth about 21% of that paper’s marks.
| Paper | Questions | Share of marks | Series |
|---|---|---|---|
| Paper 1 · Multiple Choice (Core) | 98 | 8% | 14 of 14 |
| Paper 2 · Multiple Choice (Extended) | 104 | 8% | 14 of 14 |
| Paper 3 · Theory (Core) | 47 | 19% | 12 of 14 |
| Paper 4 · Theory (Extended) | 38 | 21% | 11 of 14 |
| Paper 6 · Alternative to Practical | 23 | 21% | 10 of 14 |
Momentum’s own count, over the questions in its bank for these years: a series is one exam session, every zone included, and a question filed under two chapters counts in both. Median marks per question: 1.
Filed in Momentum under: Acids and Bases.
A worked example
In a titration, 25.0 cm³ of aqueous sodium hydroxide in a conical flask is neutralised by 20.0 cm³ of 0.150 mol/dm³ sulfuric acid from a burette, with thymolphthalein as the indicator: H₂SO₄(aq) + 2NaOH(aq) → Na₂SO₄(aq) + 2H₂O(l). Calculate the concentration of the sodium hydroxide in mol/dm³ and in g/dm³, and give the colour change at the end-point.
- Moles of H₂SO₄ = 0.150 × (20.0 ÷ 1000) = 0.00300 mol
- The equation gives 2 mol NaOH for every 1 mol H₂SO₄, so moles of NaOH = 2 × 0.00300 = 0.00600 mol
- Concentration of NaOH = 0.00600 ÷ (25.0 ÷ 1000) = 0.240 mol/dm³
- Mr of NaOH = 23 + 16 + 1 = 40, so the concentration is 0.240 × 40 = 9.60 g/dm³
- The alkali starts in the flask, so thymolphthalein is blue at first and turns colourless at the end-point
Answer: 0.240 mol/dm³, or 9.60 g/dm³; the indicator changes from blue to colourless
Written by Momentum for this page, with its own numbers; it is not taken from a past paper.
Mistakes that cost marks
- Heating the salt solution to dryness — this can decompose the salt or drive off its water of crystallisation; heat only until crystals start to form, then leave it to cool
- Using an excess of alkali to make a salt — a dissolved excess cannot be filtered off, so titrate to find the volumes needed, then repeat without the indicator
- Choosing copper metal and dilute sulfuric acid to make copper(II) sulfate — copper is below hydrogen in the reactivity series and does not react; use copper(II) oxide or copper(II) carbonate
- Saying thymolphthalein turns pink in alkali — it is blue in alkali and colourless in acid
- Treating 'strong' as meaning 'concentrated' — a strong acid is fully dissociated whatever its concentration, and a dilute strong acid still has a low pH
- Leaving a precipitated salt unwashed — rinse it with distilled water, or the soluble ions from the two solutions remain as impurities
Exam technique
- Write preparation methods as numbered steps, naming the reagents and apparatus and giving the reason for each step, and end with how the crystals are dried
- Choose the method from solubility: an insoluble salt is precipitated; a soluble salt made from an insoluble base uses an excess and filtration; if both reactants are soluble, titrate
- In titration answers, use the volumetric pipette for the fixed volume and the burette for the added volume, and give the indicator's colour change at the end-point
- Give salts their full names, including the metal's oxidation number where it can vary, such as copper(II) sulfate
Practising in Momentum
- Every question in a chapter as one list, from every paper and year in the bank, newest first or shuffled.
- Each question turns over to its own part of the mark scheme, where the board published one.
- Narrow by paper, year, session, zone and marks, and star the questions worth keeping.
- Pick chapters, a number of questions and a time, and the exam builder draws a mock to sit and mark.
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