Acids, Bases and Salts

This guide covers Topic 7, Acids, bases and salts — subtopics 7.1 The characteristic properties of acids and bases, 7.2 Oxides and 7.3 Preparation of salts — for Cambridge IGCSE Chemistry 0620 and Cambridge O Level Chemistry 5070, 2026–2028 series.

Where this fits in 0620/5070

This topic sits between the ionic bonding covered in Topic 2 and the practical techniques of Topic 12 — titration and salt preparation are exactly the kind of experiment Topic 12’s “Acid–base titrations” outcomes test you on. Understanding why a reaction produces a particular salt here also makes the reactivity-series work in Metal Properties and Reactivity click into place, since “acid + metal” is one of the reactions that topic relies on.

Syllabus coverage

CAMBRIDGE IGCSE CHEMISTRY 0620

Core

• Describing the reactions of acids with metals, bases and carbonates, and their effect on litmus, thymolphthalein and methyl orange (7.1)
• Stating that bases are metal oxides or hydroxides, and alkalis are soluble bases (7.1)
• Describing the reactions of bases with acids and ammonium salts, and the effect of alkalis on the three indicators (7.1)
• Stating that acids contain H⁺ ions and alkalis contain OH⁻ ions in aqueous solution (7.1)
• Comparing hydrogen ion concentration, neutrality and relative acidity/alkalinity using universal indicator paper and pH (7.1)
• Describing neutralisation as H⁺(aq) + OH⁻(aq) → H₂O(l) (7.1)
• Classifying oxides as acidic (e.g. SO₂, CO₂) or basic (e.g. CuO, CaO), related to metallic/non-metallic character (7.2)
• Preparing, separating and purifying soluble salts by reacting an acid with an alkali (titration), excess metal, excess insoluble base, or excess insoluble carbonate (7.3)
• The general solubility rules for common salts (7.3)
• Defining hydrated and anhydrous substances (7.3)

Supplement / Extended

• Defining acids as proton donors and bases as proton acceptors (7.1)
• Defining strong acids (completely dissociated) and weak acids (partially dissociated) in aqueous solution, e.g. HCl vs CH₃COOH (7.1)
• Describing amphoteric oxides — reacting with both acids and bases — and classifying Al₂O₃ and ZnO as amphoteric (7.2)
• Preparing insoluble salts by precipitation (7.3)
• Defining water of crystallisation, e.g. CuSO₄·5H₂O and CoCl₂·6H₂O (7.3)

CAMBRIDGE O LEVEL CHEMISTRY 5070

5070 has no Core/Extended split — every outcome above, Core and Supplement alike, is required for every O Level candidate.

Properties of acids and bases

Acids react characteristically with three classes of substance:

acid + metal        → salt + hydrogen
acid + base          → salt + water          (neutralisation)
acid + carbonate      → salt + water + carbon dioxide

Bases are metal oxides or hydroxides; an alkali is simply a base that dissolves in water. Bases react with acids (neutralisation, above) and with ammonium salts, releasing ammonia gas on warming.

Both acids and alkalis change the colour of indicators:

Indicator	In acid	In alkali
Litmus	Red	Blue
Thymolphthalein	Colourless	Blue
Methyl orange	Red	Yellow

In aqueous solution, acids release H⁺ ions and alkalis release OH⁻ ions — this is what indicators and pH actually detect, and it’s what makes neutralisation, at its core, a single ionic equation:

H+(aq) + OH-(aq) → H2O(l)

pH and universal indicator let you compare acidity/alkalinity directly: low pH (red/orange) is strongly acidic, pH 7 (green) is neutral, high pH (blue/purple) is strongly alkaline.

Strong and weak acids

A strong acid dissociates completely into ions in solution; a weak acid dissociates only partially, existing mostly as undissociated molecules in equilibrium with its ions.

strong acid (complete):    HCl(aq) → H+(aq) + Cl-(aq)
weak acid (partial, ⇌):    CH3COOH(aq) ⇌ H+(aq) + CH3COO-(aq)

Hydrochloric, nitric and sulfuric acid are the strong acids you need to recognise; carboxylic acids (like ethanoic acid) are the weak acids. Strength is about the degree of dissociation, not concentration — a dilute strong acid and a concentrated weak acid are entirely different comparisons.

Oxides: acidic, basic and amphoteric

Whether an oxide is acidic or basic follows directly from whether the element is a non-metal or a metal:

• Acidic oxides are typically formed by non-metals — sulfur dioxide (SO₂) and carbon dioxide (CO₂) are the two named examples, and both are gases at r.t.p. that dissolve to give acidic solutions.
• Basic oxides are typically formed by metals — copper(II) oxide (CuO) and calcium oxide (CaO) are the named examples.
• Amphoteric oxides react with both acids and bases, producing a salt and water either way. Only two are named at this level: aluminium oxide (Al₂O₃) and zinc oxide (ZnO).

Preparing salts

Which method you use depends on whether the salt is soluble or insoluble.

Soluble salts are made from an acid plus an excess of a solid reactant (metal, insoluble base, or insoluble carbonate), or by titration against an alkali:

1. React the acid with excess solid (or titrate against the exact alkali volume)
2. Filter off any unreacted excess solid
3. Evaporate the filtrate to the point of crystallisation
4. Allow the salt to crystallise, then filter and dry the crystals

Using an excess of the solid reactant (rather than titrating) guarantees all the acid has reacted, without needing an indicator — any unreacted solid is simply filtered off afterwards.

Insoluble salts are made by precipitation — mixing two soluble solutions whose ions combine to form an insoluble product, which is then filtered, washed, and dried directly (no evaporation step, since the salt never dissolved in the first place).

The general solubility rules decide which method applies:

• Sodium, potassium and ammonium salts: always soluble
• Nitrates: always soluble
• Chlorides: soluble, except lead and silver
• Sulfates: soluble, except barium, calcium and lead
• Carbonates: insoluble, except sodium, potassium and ammonium
• Hydroxides: insoluble, except sodium, potassium, ammonium and calcium (partially)

Hydrated vs anhydrous. A hydrated substance has water chemically combined within its crystal structure — its water of crystallisation — written into the formula, e.g. CuSO₄·5H₂O (hydrated, blue) vs anhydrous CuSO₄ (white powder, used as the standard chemical test for water).

Common mistakes

• Writing “acid + carbonate → salt + water” and stopping there. The full products are salt + water + carbon dioxide — a very commonly dropped product.
• Calling any soluble base an “alkali” interchangeably with “base.” Every alkali is a base, but not every base is an alkali — insoluble bases like copper(II) oxide are bases, not alkalis.
• Confusing “strong” with “concentrated.” Strong/weak describes how completely an acid dissociates; concentrated/dilute describes how much acid is dissolved in a given volume of water. A concentrated weak acid and a dilute strong acid can have similar H⁺ concentrations despite being opposite on both scales.
• Assuming all metal oxides are basic. Aluminium oxide and zinc oxide are amphoteric, not simply basic — they’ll react with alkalis too.
• Forgetting to filter off excess solid before evaporating, when preparing a soluble salt from excess metal/base/carbonate — evaporating with unreacted solid still in the mixture contaminates the crystals.

Quick revision checklist

• Reactions of acids with metals, bases and carbonates, and the resulting products
• Effect of acids and alkalis on litmus, thymolphthalein and methyl orange
• Ionic definition of neutralisation, H⁺(aq) + OH⁻(aq) → H₂O(l)
• (0620 Extended, 5070 required) strong vs weak acids, with named examples
• Acidic, basic and amphoteric oxides, with the four named examples (SO₂, CO₂, CuO, CaO, Al₂O₃, ZnO)
• The four soluble-salt preparation routes, and when to use precipitation instead
• The general solubility rules, from memory
• Defining hydrated and anhydrous substances
• (0620 Extended, 5070 required) water of crystallisation, e.g. CuSO₄·5H₂O and CoCl₂·6H₂O

Related resources

• Ionic, Covalent and Metallic Bonding — the ionic bonding behind salts and neutralisation
• Metal Properties and Reactivity — the metal-reactivity side of “acid + metal” reactions
• Redox Reactions — oxidation number, used alongside acid/base chemistry in some questions
• Cambridge IGCSE Chemistry hub · Cambridge O Level Chemistry hub

Written against Cambridge IGCSE Chemistry 0620 and Cambridge O Level Chemistry 5070, 2026–2028 series. Always check the current syllabus for your examination year.