7.1

Acids and bases

Cambridge IGCSE Chemistry (0620)  · Unit 7: Acids, bases and salts  · 19 flashcards

Acids and bases is topic 7.1 in the Cambridge IGCSE Chemistry (0620) syllabus , positioned in Unit 7 — Acids, bases and salts , alongside Reactions of acids, Preparation of salts and Oxides.  In one line: Bases are oxides or hydroxides of metals. Alkalis are soluble bases.

This topic is examined in Paper 1 (multiple-choice) and Papers 3/4 (theory), plus Paper 5 or Paper 6 (practical / alternative to practical). Past papers from 2022 to 2025 confirm this is a high-yield topic: Cambridge has set undefined questions worth 607 marks here (about 9.6% of all Chemistry marks across those years).

The deck below contains 19 flashcards — 3 definitions, 9 key concepts and 6 identification cards — covering the precise wording mark schemes reward.  Use the 3 definition cards to lock down command-word answers (define, state), then move on to the concept and application cards to handle explain, describe and compare questions.

Key definition

What type of substance are bases and alkalis

Bases are oxides or hydroxides of metals. Alkalis are soluble bases.

Example: sodium hydroxide (NaOH) is an alkali because it's a soluble hydroxide.

What the Cambridge 0620 syllabus says

Official 2026-2028 spec

These are the exact learning objectives Cambridge sets for this topic. Match the command word (Describe, Explain, State, etc.) in your answer to score full marks.

  1. Describe Describe the characteristic properties of acids in terms of their reactions with: (a) metals (b) bases (c) carbonates
  2. Describe Describe acids in terms of their effect on: (a) litmus (b) thymolphthalein (c) methyl orange
  3. State State that bases are oxides or hydroxides of metals and that alkalis are soluble bases
  4. Describe Describe the characteristic properties of bases in terms of their reactions with: (a) acids (b) ammonium salts
  5. Describe Describe alkalis in terms of their effect on: (a) litmus (b) thymolphthalein (c) methyl orange
  6. State State that aqueous solutions of acids contain H+ ions and aqueous solutions of alkalis contain OH ions
  7. Describe Describe how to compare hydrogen ion concentration, neutrality, relative acidity and relative alkalinity in terms of colour and pH using universal indicator paper
  8. Describe Describe the neutralisation reaction between an acid and an alkali to produce water, H+(aq) + OH(aq) → H2O (l)
  9. Define Define acids as proton donors and bases as proton acceptors Supplement
  10. Define Define a strong acid as an acid that is completely dissociated in aqueous solution and a weak acid as an acid that is partially dissociated in aqueous solution Supplement
  11. State State that hydrochloric acid is a strong acid, as shown by the symbol equation, HCl(aq) → H+(aq) + Cl (aq) Supplement
  12. State State that ethanoic acid is a weak acid, as shown by the symbol equation, CH3COOH(aq) = H+(aq) + CH3COO-(aq) Supplement
Key Concept Flip

Describe the reaction of an acid with a metal.

Answer Flip

Acids react with some metals to produce a salt and hydrogen gas.

Example: hydrochloric acid reacts with magnesium to form magnesium chloride and hydrogen: 2HCl(aq) + Mg(s) → MgCl₂(aq) + H₂(g)
Key Concept Flip

Describe the reaction of an acid with a base.

Answer Flip

Acids react with bases in a neutralisation reaction to produce a salt and water.

Example: sulfuric acid reacts with sodium hydroxide to form sodium sulfate and water: H₂SO₄(aq) + 2NaOH(aq) → Na₂SO₄(aq) + 2H₂O(l)
Key Concept Flip

Describe the reaction of an acid with a carbonate.

Answer Flip

Acids react with carbonates to produce a salt, water, and carbon dioxide gas.

Example: hydrochloric acid reacts with calcium carbonate to form calcium chloride, water, and carbon dioxide: 2HCl(aq) + CaCO₃(s) → CaCl₂(aq) + H₂O(l) + CO₂(g)
Key Concept Flip

What colour change does litmus paper undergo in an acid?

Answer Flip

Litmus paper turns red in the presence of an acid.

Example: when litmus paper is dipped into hydrochloric acid (HCl), it turns red.
Key Concept Flip

What colour change does thymolphthalein undergo in an acid?

Answer Flip

Thymolphthalein is colourless in acidic conditions.

Example: adding thymolphthalein indicator to sulfuric acid (H₂SO₄) will result in a colourless solution.
Key Concept Flip

What colour change does methyl orange undergo in an acid?

Answer Flip

Methyl orange turns red in the presence of an acid.

Example: when methyl orange indicator is added to nitric acid (HNO₃), the solution turns red.
Definition Flip

What type of substance are bases and alkalis?

Answer Flip

Bases are oxides or hydroxides of metals. Alkalis are soluble bases.

Example: sodium hydroxide (NaOH) is an alkali because it's a soluble hydroxide.
Key Concept Flip

Describe the reaction of a base with an acid.

Answer Flip

Bases react with acids in a neutralisation reaction to produce a salt and water.

Example: potassium hydroxide reacts with hydrochloric acid to form potassium chloride and water: KOH(aq) + HCl(aq) → KCl(aq) + H₂O(l)
Key Concept Flip

Describe the reaction of a base with an ammonium salt.

Answer Flip

Bases react with ammonium salts to produce a salt, water, and ammonia gas.

Example: sodium hydroxide reacts with ammonium chloride to form sodium chloride, water, and ammonia: NaOH(aq) + NH₄Cl(aq) → NaCl(aq) + H₂O(l) + NH₃(g)
Key Concept Flip

What colour change does litmus paper undergo in an alkali?

Answer Flip

Litmus paper turns blue in the presence of an alkali.

Example: when litmus paper is dipped into sodium hydroxide (NaOH), it turns blue.
Key Concept Flip

What colour change does thymolphthalein undergo in an alkali?

Answer Flip

Thymolphthalein turns blue in alkaline conditions.

Example: adding thymolphthalein indicator to sodium hydroxide (NaOH) will result in a blue solution.
Key Concept Flip

What colour change does methyl orange undergo in an alkali?

Answer Flip

Methyl orange turns yellow in the presence of an alkali.

Example: when methyl orange indicator is added to potassium hydroxide (KOH), the solution turns yellow.
Key Concept Flip

What ions are present in aqueous solutions of acids and alkalis?

Answer Flip

Aqueous solutions of acids contain H⁺ ions, and aqueous solutions of alkalis contain OH⁻ ions.

Example: HCl(aq) contains H⁺ ions, and NaOH(aq) contains OH⁻ ions.
Key Concept Flip

Write the neutralisation reaction between an acid and an alkali, using ions.

Answer Flip

The neutralisation reaction between an acid and an alkali is: H⁺(aq) + OH⁻(aq) → H₂O(l). This represents the formation of water from hydrogen and hydroxide ions.

Definition Flip

Define acids and bases in terms of proton transfer.

Answer Flip

Acids are proton donors, and bases are proton acceptors.

Example: HCl donates a proton (H⁺) to water, making it an acid. Ammonia (NH₃) accepts a proton from water, making it a base.
Definition Flip

Define a strong acid and a weak acid in terms of dissociation.

Answer Flip

A strong acid is completely dissociated in aqueous solution, while a weak acid is only partially dissociated. Strong acids have more H⁺ ions in solution than weak acids of the same concentration.

Key Concept Flip

Give an example of a strong acid and its dissociation equation.

Answer Flip

Hydrochloric acid (HCl) is a strong acid: HCl(aq) → H⁺(aq) + Cl⁻(aq). The single arrow indicates complete dissociation.

Key Concept Flip

Give an example of a weak acid and its dissociation equation.

Answer Flip

Ethanoic acid (CH₃COOH) is a weak acid: CH₃COOH(aq) ⇌ H⁺(aq) + CH₃COO⁻(aq). The double arrow indicates partial dissociation, and that an equilibrium is formed.

Key Concept Flip

If 20.0 cm³ of 0.0150 mol/dm³ Ca(OH)₂ is neutralised by 25.0 cm³ of HNO₃, what is the concentration of the HNO₃ in g/dm³? (Mr of HNO₃ = 63)

Answer Flip

1. Moles of Ca(OH)₂ = (20.0/1000) * 0.0150 = 0.0003 mol. 2. Moles of HNO₃ = 2 * 0.0003 = 0.0006 mol. 3. Concentration of HNO₃ (mol/dm³) = 0.0006 / (25.0/1000) = 0.024 mol/dm³. 4. Concentration of HNO₃ (g/dm³) = 0.024 * 63 = 1.512 g/dm³ or 1.51 g/dm³ (to 3 s.f.)

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6.3 Redox 7.2 Reactions of acids

Key Questions: Acids and bases

What type of substance are bases and alkalis?

Bases are oxides or hydroxides of metals. Alkalis are soluble bases.

Example: sodium hydroxide (NaOH) is an alkali because it's a soluble hydroxide.
Define acids and bases in terms of proton transfer.

Acids are proton donors, and bases are proton acceptors.

Example: HCl donates a proton (H⁺) to water, making it an acid. Ammonia (NH₃) accepts a proton from water, making it a base.
Define a strong acid and a weak acid in terms of dissociation.

A strong acid is completely dissociated in aqueous solution, while a weak acid is only partially dissociated. Strong acids have more H⁺ ions in solution than weak acids of the same concentration.

Tips to avoid common mistakes in Acids and bases

More topics in Unit 7 — Acids, bases and salts

Acids and bases sits alongside these Chemistry decks in the same syllabus unit. Each uses the same spaced-repetition system, so progress in one informs the next.

7.2 Reactions of acids

10 flashcards
7.3 Preparation of salts

12 flashcards
7.4 Oxides

10 flashcards

Cambridge syllabus keywords to use in your answers

These are the official Cambridge 0620 terms tagged to this section. Mark schemes credit responses that use the exact term — weave them into your answers verbatim rather than paraphrasing.

acid base alkali pH pH scale indicator universal indicator litmus hydrogen ion H+ hydroxide ion OH- neutral acidic alkaline strong acid weak acid hydrochloric acid sulfuric acid nitric acid sodium hydroxide ammonia

Key terms covered in this Acids and bases deck

Every term below is defined in the flashcards above. Use the list as a quick recall test before your exam — if you can't define one of these in your own words, flip back to that card.

What type of substance are bases and alkalis
Acids and bases in terms of proton transfer
A strong acid and a weak acid in terms of dissociation

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